Downgrading iPhone 3GS to a version I never used.

I used to use the iOS 4.1, then I upgraded my iPhone 3GS to the iOS 6.0. Can I Downgrade it to the 4.3 version? or 5.1.1? Or should I downgrade it to the last version I used? I'm downgrading it because it is locked. I jailbroke it, but it still with no Signal to my SIM card. I bought it in another country, 2 year ago and never updated.