No, just no. Fallout 3 has no code set up for any kind of online interaction and would require a lot of work that would be better used for Elder Scrolls 5 and Fallout 4.
However, a Fallout mmorpg is underdevelopement at the hands of Interplay. Interplay currently holds the rights for any kind of online game, though if they don't produce something within a certain time span the rights apparently move back to Bethesda who purchased them outright a few years back.