Title says it all

If it's strictly for personal use, it should be fine. Bethesda doesn't allow TES games to share content, so I'm guessing that the Fallout series is the same way. I'd contact Gstaff just to be sure though, not that anything would happen aside from you being asked to removed the mod(s) anyway.

I found a New Vegas mod that links it with Fallout 3. Since this sounds like Morroblivion, I don't think I can post a link, but the name is A Requiem for the Capital Wasteland. If that's what you're looking to do, it'll be much easier to use that mod. If you're just looking to include something from FO3 in your game, note that you can't redistribute Bethesda assets unless you've modified them in some significant way (changed color, size, etc.).