hi flipflop,
we took our 3 at 3 y.o. (actually there for their 3rd bday...gotta love the free tickets and buffets for the under-3 set!) and 3.5 y.o, and there really wasn't too much of a difference as far as schedule, interest in rides, etc.
the only big difference may be the potty training issue (unless your's will be potty trained before 3, anyway)...at 3, mine were still in diapers, and at 3.5, although they weren't really wearing underwear ALL the time at home (pullups at night and when we went out), we found that pullups all day long were better for the kids while in the park. i didn't feel like they were "going backwards" on their progress, and they got back into the swing of things pretty easily once we got home...i am also someone that PREFERRED diapers in the parks---it was SO much easier than herding 3 little ones to the bathroom!
if i remember correctly, my kids might have had a tad bit more stamina in the parks at 3.5 y.o., but really, they still absolutely needed naps and they were beat by their normal bedtime (8:30-9) on both trips.
the trip we took when they were 3.5 was a bit more challenging than the earlier trips (at 2, 2.5, and 3 y.o.), but that's because they themselves were a bit more challenging (more opinionated, more independent, more whiney). when they were a bit younger, my kids were happy doing WHATEVER we suggested, but they were very laid-back toddlers, for the most part...i think the thing that you've got going for you, at any age, is the fact that they will all be interested in the same stuff, developmentally (i can do 3 two year olds- no problem-, but i can't imagine keeping a baby, toddler, and preteen, for example, happy all at the same time at WDW!)
we've gone late april/early may and nov. for the last 3 years , so here are my thoughts on may v. dec:
may:
PRO:
flower and garden festival at epcot (great music, incredible topiaries which your kids will LOVE, butterfly garden, and at least 2 playscapes which will be perfect for your little ones)
lovely swimming weather, ESPECIALLY for the kiddie pool at BC...we stayed there this past May and we swam EVERY day (we've never done that before) because that sandy-bottom pool is SO SO great for little ones.
CON:
more crowded (as compared to our nov. trips, anyway)
can be hot, hot,hot
dec:
PRO:
incredible seasonal decorations...really gets you in the mood for the holidays
cooler, less humid weather seems easier on the kids (although in dec, it will probably get quite chilly in the evenings, which means packing/carrying more gear)
CON:
the $$$ (this alone might do it for us!)
the waiting (that's an extra 7 mths!!! that's a LONG time to be patient for an incredible vacation)
not very comfortable swimming weather...this is a huge factor for us
not sure this helped, but if it were me, i'd choose May for the savings and the swimming!
oh ,as for the bugs...we go the first week in may and haven't had any problems with them (mosquitos or love bugs)
good luck deciding,