Not annual but Sempervivum comes to mind. For something a little bigger, maybe pansies, Dianthus, or cabbage/kale.

With only 3 inches of soil to work with, stick with Sempervivums (hens and chicks), stonecrops or other equally shallowly rooted plants. No way will cabbage or kale survive in only 3" of soil!!

I've never had much luck with sempervivum living over the winter :-/ It could be because the last few years have had warm winters with that one sudden 15 degree week? But I'm getting kinda tired of spending $100 every year on new ones.

I thought about pansies, mainly because I can get a 6 pack for like $4 at Lowes right now. But do they live all Winter, or just through the Fall? I'm getting mixed information on that.

Are you sure you are getting sempervivums? They are winter hardy down to zone 2 so should have no issues surviving winter if drainage is good.

Pansies are a cool weather plant and if planted now should continue to grow and flower (if deadheaded) until things start to heat up in spring. Extreme cold can stop flowering temporarily.

I mean, they LOOK like sempervivum! LOL But I have no way of knowing whether they're some sort of hybrid or something.

Last year I filled it up with what were labeled as Blacks, Royal Rubies, Cobweb Buttons, and then added a Blue Heron echeveria, Ice Plant, and Lemon Ball. I moved those last three to the greenhouse and left the first 3 in the pot, but they all died.

For drainage, I drilled about 20 holes through the cement (I think the bit is 3/8", but it could be 1/4"), and the soil is a mix of potting soil and perlite (I think I did 60% soil 40% perlite, but I can't swear to that). Everything seemed great through the warm months so I don't think that drainage is an issue?

I didn't buy new sempervivum this year, I just put the Lemon Ball and Ice Plant back in. They all did well, but it's getting close to time for me to put them in the greenhouse.

What about something permanent like Aubretia or Mossy Saxifrage? Spring flowers and green the rest of the the year.