u must install a custom 4.1 that has been set to activate iPhone (use sn0wbreeze and go to advanced u will see the option there)
Then go install that custom ipsw to ur iPhone , the only issue with that is u will have a battery drainage problem , every few days u will need to go to settings general reset and reset all settings , this will fix the battery issue but only for a few days u will have too do that every few days if u dont want ur battery too die within a few hours !
that OR
find the original sim card that goes with that iPhone and activate it like that !
Go get an unused official sim for your iphone or when building custom firmware choose activate iphone and restore again.
Check your baseband for your 3g iphone