Cushions can be tethored in different methods. Also important for longterm comfort is the way they are filled. Pocketed springs, urethane foam and the different densities, and latex in different densities. Urethane will deteriorate the quickest.

2 cushion looks nice, but 3 does make for better group seating as you mention.

For a few times a year, please yourself and the dog!

I have a two cushion sofa that is almost five years old + has a lot of use. My down wrapped cushions look just as nice as they did when I purchased it. Every 10+ days I fluff (= slap the cushions with my hand a couple times) and flip the cushions + alternate sides of the sofa. I also switch and turn the back cushions (also down wrapped). It doesn't take me more than five minutes to do all of it.

One side of the sofa is used by me every night + several times during the day (I have some health issues = there are days that I have to take it easy = I use the sofa to read/nap/etc.) and the other side is used by my SO every night for a few hours.

My sofa isn't a lower end budget sofa - it's made well.

The sofa I had prior to this one was also a two cushion sofa - it lasted many years in my house + was used by my daughter for a couple years at college. I gave it away a couple years ago - the cushions weren't down wrapped and looked a little bit out of shape at that point - but the person I gave it to added new inserts and it looks great again.

I like a one cushion sofa with a tight back. It eliminates the problem of gap between seat/back cushions that can be uncomfortable if you need seating for three on a two cushion sofa. I like the sleeker look, as well. Comes in a bunch of styles to fit your style.

I agree with chispa that a 3-cushion sofa holds its shape better than a 2-cushion sofa, even when using good quality foam and down cushions. I gave up a 3-cushion sofa for a boxier 2-cushion sofa because I thought the rolled arms look dated, and I rue that mistake every day. 😢

For your everyday use, I wouldn’t rule out a two-cushion sofa just because you occasionally seat three. I’d make your 20–22-inch usable seat depth the first filter: measure from the front of the seat to the face of the back cushion, not the sofa’s overall depth. If you can try one in person, sit in your usual TV-watching position with your feet on the floor.

For the popping-up concern, try sitting near each cushion’s inner edge, then across the centre seam, and have someone sit beside you. Ask whether the cushions are attached, reversible, and replaceable; those are more useful things to check on the actual sofa than assuming two cushions will behave a particular way.

A bench seat is worth trying for comparison, as Judy suggested, but I’d still test the middle position. Which two-cushion models are on your shortlist?

I love it when there are lots of pillows.