That depends - some does will produce three, some two, some one, and some won' t produce any. Some does even grow antlers (not unusual). It depends on a variety of factors, but the three big factors IMO would probably be - in no particular order - 1- Diet - nutritional requirements are met/exceeded in order to be able to sustain enough energy for 4 deer (1 adult doe and three growing fetuses through full term); 2- Genetics - this goes for all involved (buck, doe, fawns); 3 - Chance - there could be any number of other factors, whether seperate or combined that would effect the probability of such an occurrence. If I see a doe with triplets, I automatically assume that the area is at or below it' s carrying capacity.