You do realize that you can get SBRs in Illinois now, right? The law changed recently.
As for me, 16", then 11.5", then 18" for a third.
10.5", 16", and 20"
12.5, 14.5, 18
I personally like the 14.5... i think it is a pretty versatile length. There are a few applications that i would like something really small like the LWRC UCIW in 6.8 but for the most part w/ 556 12.5" is really as short as i would "need" it. I guess i am one of the oddballs that is happy with 14.5 tho. Anything that i would need an 18" to go out and get, i would ideally have a caliber other than 556.
I would narrow it down to a 18" barrel with rifle gas system and 12.5 SBR with a can.
If I could only have 2, I'd prefer a 16" Midlength and a 10.3" Carbine. If I could have a third, I'd want an 18" 6.8 SPC.
11.5" Carbine and 16" Midlength and a can.
... and this is what I actually have, so there's got to be something to it. That said, I'd run an 11.5" just as happily.
For the third, a generic 16" one wouldn't mind feeding crap ammunition to is what I'd add to it - if it runs and feels just like one of the other two extra success, they also make great beater guns and spouse weapon systems.