Dave Meltzer is reporting that the third UFC on Fox event will take place on May 5. That is the same date that Floyd Mayweather, Jr. has blocked out for his next fight. Despite all the bluster from Floyd on Twitter yesterday there is almost no chance of the fight that date being against Manny Pacquiao. The next best option would be for a bout with Saul "Canelo" Alvarez but the reported negotiations with Robert Guerrero probably mean that's what we're more likely to see.
Regardless, the UFC will probably take the same tactic that they did when they aired the UFC on Fox show against Manny Pacquiao vs. Juan Manuel Marquez and insist that they'll be off the air in time for the Floyd fight.
If it is Mayweather against Guerrero it's much less important to play that up since it won't do half the number of buys as the standard Floyd or Manny show with "The Ghost" having no name value whatsoever.
No word yet on what the target main event would be for the UFC on Fox 3 card. Cinco de Mayo weekend is traditionally a big fight weekend in boxing where promoters have looked to put on big cards featuring Latino fighters. Given Cain Velasquez's loss on the first Fox fight, I doubt they'd try to use the same tactic with his rumored bout with Antonio Silva.
Update: Bloody Elbow has been able to confirm this news. The target date is May 5.