The common belief and when you google how to unlock 3GS is that you have to have a old bootrom which is compatible with iPad BB, new bootroms aren't compatible with iPad BB.
But this is not true, the one way to for sure tell is to open your 3GS and look at your baseband chip.
I posted a question on this forum asking about my new bootrom being compatible with iPad BB, and thanks to J03Y4G giving me pointers and such I found out some information from there, your bootrom doesn't necessarily matter.
Open up your iPhone and look at your baseband chip, if it is a EE chip not a EF or EH chip. The EE chips are compatible with iPad BaseBand. EF and EH are NOT. The bootrom doesn't matter.
TO VIEW YOUR BASEBAND CHIP REMOVE THE BOTTOM HEAT SHIELD AND IT'S ON THE BOTTOM LEFT.
EE - Could have old or new bootrom but IS compatible with iPad BB(If it has a new bootrom and it's an EE then you more than likely have a refurbished model but is compatible with iPad BB.)
EF & EH - Will have new bootrom and IS NOT compatible with iPad BB, do not flash iPad BB