Just a caution that "topsoil" is the lowest grade soil. It's really just scraped off the top of something. Yes, it doesn't mean anything beyond that. It's kinda brown. Landscape supply houses and nurseries will usually stock soil blends in bulk. This is higher quality stuff made with compost that also probably has some quality control about debris and rocks.

We have relatively deep raised beds we built three years ago, and I used the soil proportions stated in this article from Farmer's Almanac (scroll down to the section titled "Filling a Raised Bed"). Our plants have done great in this mix.

If you haven't yet built the planters rethink the depth. This ARTICLE (scroll down to the list) says the majority of plants do not need more than 24" deep soil. Most container plants do better with wider pots rather than deeper.

Regardless of what you decide with the planters, you are correct that the names for soil do not tell you much. Topsoil is the uppermost layer of soil and most fertile due to the organic content, but there's not a surplus to be sold off. Garden soil refers to a mix of soil, coarse sand and organic matter in more or less equal amounts. Any mix that contains soil is going to be heavier than potting soil because potting soil doesn't usually contain more than trace amounts of soil.

The basic soil used in garden soil is not guaranteed to contain all the necessary nutrients for optimum growth. The organic content helps mitigate any deficiency, but you should still be on the lookout for possible problems. I would use a regular application of an organic liquid fertilizer that contains micronutrients to ensure productivity. Here is a PDF that explains micronutrient importance.

Where the organic material comes from is important. Some sources could use composted sewage which may be contaminated with excess pharmaceuticals and chemicals. Much better if the compost comes from recycled yard waste or lumber mills. Here is a LIST of compost sources in Santa Clara Co. Find out which are most organic. You could just fill your containers with compost. You would definitely need to use a complete fertilizer, but you would also avoid any bacterial or fungal disease common in soil. Whether or not you choose soil or compost, you will need to top off your containers with new compost once a year. Both a soil mix and compost will settle over time.

Yep. Totally agree that you do not need three feet.

Thanks for all the comments, and the great links I realize I didn't explain why I need the 3 ft...

The reason I'm building raised beds is to get access to additional sunlight. I have limited places for a garden in my back yard. At ground level, the available space that gets full sun is significantly smaller. The higher the plants are, the more sun they access. (And it's an all-or-nothing sort of thing; once I build raised beds in one place, I need them throughout the area, because they cast a shadow.)

Dealing with the difficult soil is really just a side-benefit. Another perk is easier access; right now I often find it a bit awkward to access wide rows when they're at ground level, and I keep worrying that I'll fall into the row and smoosh the nice aerated soil. I guess it also makes it a lot less likely someone else will accidentally walk into my vegetables also!

The thing I'm concerned with is that if soil in the lower portion of the bed compacts (due to the weight of a couple feet of soil and a good watering), it might act as a "stopper" that prevents drainage. The same thing would happen at ground level too, except that the water can percolate out laterally on the sides; in the confined space of the planter, the only way out is down.

I might be overthinking this. But given the expense of these things, I don't want to miss my chance to get this right. I definitely don't want to have to deal with digging up the soil later. So if I can just throw in some perlite or sand or something in the bottom layers to avoid it, that's what I want to do; but I'm not confident that wouldn't make things worse somehow.

You don’t need to fill the whole depth with your soil mix. Here’s a useful article https://www.epicgardening.com/how-to-fill-a-tall-raised-garden-bed/

We put in a 3' tall raised bed about 5 years ago. I had equal amounts of topsoil and compost delivered, and tossed in random things like potting soil from previous years potted plants, a bale of straw, pinecones laying around the yard, etc. It did settle quite a bit, so every year, I add compost, and more old potting soil. It is a very productive bed.

I'd fill the unnecessary depth with something lightweight. I've use huge overturned plastic pots, empty recyclables like milk containers or detergent bottles in the bottom of large containers that I empty in the fall. Two years ago, I filled the bottom of my overlarge clay pots with punky logs.

If I were growing annual crops or flowers, I'd build a hollow, false bottom. Three feet of soil would be a terrible, unnecessary strain on your raised bed walls, I would think.

Here's another video for building a new raised bed more economically:

https://www.youtube.com/watch?v=y2kHArkO_Bw

I began building raised beds more than 25 years ago and modifications have been ongoing over the years. I make compost for my use and buy it by the yard when used on a customers yard/garden. If people grow gardens in their native soil,I find mixing it 50-50 with compost is as good and often better than bagged garden soil. If native soil has really poor drainage,I some times replace 10% native with sand but for the most part,50-50 drains very well. I began making beds extra tall as my ability to bend and stoop became limited. To fill a 3'bed I use logs and limbs and fill space between them with leaves and ground mulch topped with 2-3 inches sand for lower 2' then remaining 1' plus 6" mounded above with 50-50 mix. I have expanded this system to build 5' tall x 8 to 12' diameter mounds where trees and other perminate plants will be planted a few years later. If i desire a compact bed with retainer eventually,I inclose brush pile with aged or fast to rot logs. To make it atractive while rotting down,I plant annuals,vines,ground cover or ortimental tall grass. To get a better Idea of what to expect and how to go about this, look at "Hugelkultur" and "Keyhole Gardening".

As for drainage,I reccomend inclosure not be water tight. One such inclosure might be boards that water could seep between. Even better and more pratical is having several "cups" or protrusions exposing soil where plants could be grown in the fashion of Strawberrys. Either of these will not only provide drainage but supply much needed air in soil deep down.

You can save lots of expense buying in bulk apposed to bagged. I pay from $20 for shredded limbs to $45 for colored mulch and compost by the yard. Pool companies will give dirt away when digging new pools. Just make certain there's not some undesirable rock or soil at bottom of excuvation. Soil from more than 3 feet down is pretty sterile but will come back strong after mixed with organic matter and aged a few months.