hi,
I've used this before: **broken link removed**
Although it requires a minimum of 5v on the input and can only provide 90mA out...its a start. I'd suggest using CMOS 4069 for the chip, and adding a MOSFET driver. As long as the Inductor can cope with 5A+ continuous, it sohuld work fine, and its a pretty small circuit that doesn't require any wound components...as well as providing some regulation. I know you said you didn't need regulation...but otherwise it'll swing between 8 and 15v (thats what mine did). Its also surprisingly efficient for what it is, and small.
Well I tihnk I'm done selling that idea, gotta love EDN's design idea's. An alternative would be to use a 555 timer, and a cockroft -walton multiplier...althoughagain it'll need a MOSFET to drive it, and is probably a waste of time regarding efficiency. I didn't want to repeat what others have said, they're pretty good idea's, especially for the quick and dirty approach.
I must say though, its gonna be tricky working at 3v 5A...I didn't know 3v bats could kick out that sort of current...internal resistance...
Blueteeth.