It sounds like the 6 yr old still needs a harness. So I would keep him in the Frontier and do either #1 or #2 depending on how much I wanted to keep the 3.5 yr old rear facing. He is totally safe forward facing and you have done a great job RF! But since you asked opinions...personally, I would buy another convertible and keep the 3.5 RF until the baby needs the seat (or even a few more months then switch FF, either way)and then buy a dedicated booster for your oldest, give the Frontier to your middle son, and the new convertible for the baby.
DD1 9 yrs old 12/2011
DD2 7 yrs old 01/2014