I think that guppy fry are actually a bit smaller at birth than platie and molly fry. Probably the most certain way of telling what species the fry are is to take a look at the mothers. Whichever fish is the skinniest, probably gave birth to the babies.
mollie will be the larger of the lot and be quite obvious to see, like in the picture kindly provided above
guppy are grey/clear when born and have a very pinched rear end when viewed from the top, platy will be born with thier base colours and will slightly fatter and shorter in lenght than the guppy. hope this helps