I'm taking power from a 100 amp subpanel in an outbuilding and running three branch circuits: two to small outbuildings for lights, one to a horse waterer that draws 1.5 amps. Two questions: can I run the three UF wires in a single trench for the first 50 feet? Do I bring all three UF wires into the power source building in a single LB or do I put in three LBs?
IME, IMHO, UF is a terrible idea, but you can certainly put 3 UF in one trench, and you COULD bring them in through one (HUGE) LB - the LB needs to be HUGE because oval wire is treated as round wire of the largest dimension of the oval for conduit fill purposes, and 3 wires does not get the one-wire higher fill percentage. Also practically speaking you'll appreciate having the space to get the job done with less swearing.
I would STRONGLY recommend conduit and individual (wet rated) wires. The trench is the expensive part. Conduit means never having to dig the trench again, while direct burial means you might have to do that. Not infrequently conduit and wire is competitive with or actually cheaper than direct burial wire.