Can you tell us where you are located? And how big the containers are? All three of your choices have the potential to grow quite large, especially the Rosemary, in the right climate.

Oh sorry...6b/7a and the container isn't large...perhaps 10 inch diameter, 8 inches deep. I've grown rosemary and sage for years as annuals and they've never grown big. I wanted to grow them as perennials but haven't managed to do so.

A single plant of any of those will easily fill that size container. And the rosemary will rapidly outgrow it. I'd do a separate container for each. And if possible, one that was a bit larger overall (at least deeper) for the rosemary.

Well, I have tried sage and rosemary before in large pots, and they never grew. So this year I am experimenting with growing conditions. If they get too big I will just move them.

Sage and rosemary are shrubs - their growth is by nature slower than that of herbaceous herbs. But I'd suspect your soil conditions may not be suitable, resulting in stunted growth. These want a very textural, fast draining soil without a lot of added nutrients or organic matter.

Also, if improving your soil conditions results in better growth - and I'm sure it will - it will be very stressful on the plants to later try and separate them or move into separate containers. Make things easy for yourself by potting them up separately now!

So what should I do to the potting mix to make it more suitable?

Since I don't know what you used and there is a big difference with different brands, I can't be too specific : -) What you are shooting for is some decent particle size - not the small, fine, peaty stuff like in Miracle Gro soils. Add some bark fines and perlite/pumice or grit in about a 50-50 ratio to the existing potting soil. If you can't locate bark fines (often sold as 'soil conditioner'), fine orchid bark works perfectly well.

Hm...I am using a Miracle Gro, actually: http://www.miraclegro.com/smg/goprod/miracle-gro-potting-soil/prod70332

It seems to me to be very dense. I have a bucket of sandy soil taken from our house near the beach that I thought of mixing in.

No!! No "real" soil and no sand. Just the ingredients I listed.

Won't bark fines also acidify the soil?

No they won't.

I live in zone 6b as well. My sage is a perennial although not a long lived one (my fennel is 20 plus years old -- the sage needs to be replaced every 5 years or so). The variety of rosemary called Arp is more hardy in zone 6b and, of course, microclimates in gardens make a difference. I've not had an Arp, but I did have one rosemary for 6 or 7 years before I lost it one really cold winter. Lavender is my area the variety Provence seems to do the best and I've had that live for a long time.

I have lost rosemary and lavender in winter but mostly it wasn't because of the cold but because of the dry.

But all my plants are in the ground and that DOES make a difference. I think winter kill is because the roots get too cold in a pot. Think of yourself in a hot tub in the winter, you're warm and fine because you're surrounded by warmth. Now get out and it is COLD because you have nothing between you and the cold.

Sometimes if you have a garage you can bring them into that will work/help to overwinter them.

Oh, I know microclimates are everything! However, my sage & rosemary barely grew at all during the summers. That is why I am trying something different this year. If they overwinter, I will reassess the situation. But for now, I'd just like to get through the next 4 months with some growth!

As Gardengal pointed out, sage and rosemary are shrubs and their annual growth rate is consequently not enormous. If they put on 2 or 3 inches that would be a pretty good result for their first year after planting. You don't actually want to push them to produce a lot of soft sappy growth if they are going to overwinter successfully. They need to be grown relatively hard. Did yours not grow at all?

Now that I did not know. In the past, they grew...I guess, not much more than 2 or 3 inches.