3 cft is a small box for the 15 where as 3.5 is a real solid box for 2 10's 2.5 cft would be the small box for 2 10s but i wouldnt suggest a small box for a solobaric
less cone area. look up the solobaric manuals on stillwater designs web site to get box details. 3 8's would be killer too lol i have a friend with 1 in a 1.25 cft box it out humps alot of 12's lol
http://www.kicker.com/05/05kicker.html hit the support section then manuals take either the l5 or l7 (box specs they recomend are the same only difference is power handeling