I'm not an expert at this, but they should work if the bands are the same. They locked out the AT&T bands on the Verizon Droid 2, im not sure about the droid 3.
Droid 3 supports - All–Digital, 800/1900, CDMA EVDO rev A and GSM 850/900/1800/1900 UMTS
Found on -
Droid 3 will work on FOREIGN sim cards when unlocked, but will not work on AT&T USA and T-Mobile USA. I read somewhere that they are making a Droid 3 or something similar for AT&T though. I left links, from where I found this information if you would like to see for yourself