It's funny that anyone ever asks questions on this board anymore since the advice almost always comes down to the same thing: "Call your schools and ask..." 😉
Anyway, the first thing to do is check the requirements for each school you're applying to. As mentioned, some have very clear requirements ("one vet, one prof, one employer" and "two vets, one prof" seem to be the most common) whereas others require "at least one vet" or something more flexible. And, some schools throw out any letters beyond three. Ideally, then, your combination of schools has overlapping requirements and you could satisfy them all with three letters. If you happen to be applying to both a vet/prof/boss and a vet/vet/prof school, and especially if either promises to trash extra letters, call each of them and tell them that you're applying to another school with different but equally strict requirements, say you're sending letters from two vets, a prof, and a boss, and ask if they'll make sure to use the three that are relevant to their requirements even if one of them is on a paper form.
You're paying a lot of money for each school you apply to, so it makes no sense to just send in a combination of letters and hope everything works out.