The way to deal with 2-3 min sets in my experience (I'm lucky enough to have enough choices that I often don't have to worry about this) is to have a solid 1 liner opener. You don't have to be a 1 liner comic but chances are you've all got at least 1 good 1 liner. Then work on shorter bits or parts of chunks (what I should've done the other night when I went over and irritated the host) that are 1-2 min long.
If you need structure then go listen to some people who have bits akin to this (or parts of longer ones) and write out what they say to give yourself a feel for how quickly they move around and have punchlines.
That being said I usually avoid open mikes that don't have 4+ minutes but there are usually enough independent shows that I can do my 7-8m there and hit up an open mike and work on a diff 3m (of 4 after an opening reliable one). One thing that was harped to me early was always have a goal or purpose behind every set. And that doesn't just mean 'say this joke this way, follow this order, etc' it can be as simple as standing up straight, not looking down, and maintaining eye contact (or the illusion thereof) with the audience.
Early on it feels boring cuz you don't always understand how many levels there are to work on. I feel that way and it took me 2 1/2 years to get a 7-8min act o.o