With a 3" thick cushion, the two piece method will give you a more rounded edge and difficult corners.

Although a little harder, I'd go with the top, bottom and sides method. It will look more professional.

Remember to put a zipper in for removal.

Is this an existing cushion or did you purchase foam to make one? If you are recovering I like to remove the existing fabric (carefully) and use it as a pattern. You can reuse the cord for the piping too if you take that apart as well.
Do you have a photo of the bench cushion?
With the three inch thickness I would do the 3 piece. Piping is not difficult to apply. If no pattern is available to copy you need to cut the pieces with seam allowance. The cover should fit snugly. Do you want a zipper or not? I am guessing you are using decorator fabric which is not washable. A layer of batting is often applied over the foam before covering.
Hope this helps. Let me know if you need more info.

Here is a link that might be useful: Piping info

dian, dragonfly, dlm, thanks very much for your replies. I'll do the box cushion (so that's the name for it!). If piping hides mistakes, then I'll suck it up and give it a try. I was thinking of not putting a zipper in (in fact, there's no way in heck I could do it) but rather sewing up a seam on the back with blindstitch instead. Would that work?

This is not an existing cushion--I'm making it from scratch. I love the line "the cover should fit snugly". Well, yeah, but many's the slip twixt the cup and the lip :) But in the effort to make the cover fit snugly, if the cushion is those dimensions, should I make the cover precisely those dimensions plus 5/8" on each side for seam allowance? Or do I make it smaller? I want it crisp but not looking like it grew out of last year's cover, iykwim. No slack, though. I was just going to use foam, no batting, so it would be crisper. Or is that wrong?

Zipper is optional. I have done it both ways. If you do not plan to remove the cover to wash then it really does not matter. No one will see it if you hide seam in the back. For your first cushion make it as simple as possible. You may want to experiment on the next one.
The batting is not necessary it is just adds to a nice finish to the cushion. If you notice sofa and chair cushions will have this over the foam.
Measure foam all sides. Each piece will be cut separately. At each seam add seam allowance, no extra. This will give a snug fit without being too big or too small. If it is too small it will crush the foam and distort.
Good Luck! Love to see it when you are done.

I'm worried that the fabric might stick directly to the foam? Batting applied to the foam helps the fabric "slip" across the cushion so you don't get those weird wrinkles after someone sits on it.

For sure I would make a "mock" cushion first out of cheap fabric to get your sizing down. I would tell you to use muslin, but it's much softer and pliable than your decorator fabric, and will pull differently over the cushion. So try to find a cheap fabric that is the same weight as what you'll be using.

Make yourself templates out of brown paper and mark them so you know which size you're trying out.

I do like the look of the box cushions with coordinating piping. It's not hard to do---remember to sew it on the bias! Lots of info on the internet how to do this stuff---it's really pretty easy and self-gratifying.

flyleft you can do it!! I hesitated as well. For wicker loveseat in my DDs room- we made a pattern out of paper towels taped together and traced the outline of the seat. cut that out. pinned onto fabric. measured 1/2 inch seam allowance all along the pattern marked with ball point. cut exactly on the drawn line. Since we used a 2 inch foam- we measured corner to corner of the foam. added 1/2 inch to all sides. so we had 4 -3 inch by whatever pieces. Sewed the strip together. ( we knew we were not doing a zipper -google this part if you are doing zipper-look for box cushions) Pinned the seams of the strip to each corner and then pinned the rest. sewed the top all around.Clip the corners. sewed the bottom leaving room to shove in the foam. hand sewed seam closed. use a crochet hook or pen to poke out the corners nice and sharp. It fit like a glove. I can take pics if you like. we had never done it so I googled making box cushions. I cut the foam to fit on my DHs band saw!!( I had sold my electric knife at a yard sale years ago not knowing it was good for foam~
We did not do piping although I dont think it makes it harder, just more time consuming we were in a hurry as my DD wanted to design and sew pillows for her room
My loveseat on my porch I just sewed the two pieces together allowing for the stuffing. I prefer the box cushion. I was in a hurry to be done and use it. I have enough fabric to do it again with foam and piping. HTH Sue

What happened to my reply?! I replied to you all and now I come back and see it and it didn't make it. Ach. I must have clicked off the site before actually clicking through preview. I'm sorry.

ANYWAY, thanks for the further information. I do wonder about the batting--if it's needed to prevent what texas is talking about--the stickiness thing. Texas, the practice idea sounds very good. Not so good for my impatient self, but better for my perfectionist self :)

Now that I really read through the piping instructions, I'm thinking that this fabric is too thick to use for piping...maybe I should find some thinner coordinating fabric? This is pretty tough commercial stuff, 100,000 rubs or whatever, iirc...the idea of bias cutting makes my throat get all tight, but I'll give it a try.

cooperbailey, you're amazing--this is *great* detail. (and no, I'm not doing a zipper. And I bought a knife from ebay to cut the foam--it was really fun and I was sad it was over so soon :)). So you didn't use batting? Your instructions really help me visualize it. I will try it this weekend with some practice fabric and use your instructions, along with the piping instructions...

Thanks, you all! I'll post pics when I'm done with the real thing.

I just had a cushion made for my bench and I had them put 3 buttons (tufted or what ever it is called) spaced evenly down the center . It makes it look more finished

[sticking fingers in ears] la la la la...I don't hear you...

:)

flyleft- two things- first if you have curves on your bench you will need to pin really well and ease it in..
We tufted a pillow with 2 buttons, button thread and a 3 inch doll needle. sew thru the batting- like you are sewing on a button.You can do it too. Probably just use a button on top. I dont know how well it works with foam but it worked well with batting. Give it a try!
Sue