I don't think SFC2 and SFC3 are vastly different, although I've never played bridge command so idea on that one, but SFC2 and 3 are both naval simulators. Both share many common tactics and stratagies. Weapons pretty much work the same way. There are differences, but mainly the difference is in the "feel" of the games. I think the games are close enough that should the source code for SFC 2 OP get released, that with a few subtle changes, SFCOP could win over the SFC3 crowd.
One of the things I like about SFC3 are the interfaces. The interfaces for both commanding a ship and on the dynaverse are far "cleaner" and far more intuitive. Being an old SFBer, I just can't get over warping in combat.. ick. I can do it, but I don't think I will ever like it.