In my humble opinion:
A/B: You already have two DVMs, so an academic reference is an opportunity to provide admissions with information about you in the classroom as opposed to in a work environment. Even though our end goal is to be successful in a clinical setting, we still have classroom time that we’ll need to do well in to get to that goal, so a professor’s perspective is a great way to show that you’re capable of that . . . but only if the professor can actually explain why you’re capable of that. I ran into the same issue with my own recommendations, since I’ve only taken a few classes this past year; all of my professors either haven’t seen me in years or only saw me for one semester.
What was your communication like with Professor A when they wrote you a letter for VMCAS? Did they seem to remember you well and seem eager to write you a letter, or did you have to remind them who you were? How long have you known Professor B and how much one-on-one interaction have you had with them while taking their class(es)? Do you feel that you’ve had the chance to demonstrate to them that you’re a dedicated student and that their class matters to you?
If either one of these two professors seems to know you well and see you in a positive light at this time, I think that would be a good option for your third slot.
C: While different from an academic reference, a vet tech also has a useful perspective. They know what makes a good veterinarian and thus can point out qualities you have that line up with that. Since you’ve already gotten positive feedback from this person and have worked together for over a year now, I don’t see how you could go wrong with this one.
D: It’s great that you were able to make a good impression in a relatively short time, but I don’t see how this one will add anything useful to what the first two DVMs (who have presumably known you longer) can say about you. I think this is a good backup in the unlikely event your second vet says no. Two DVM letters are a great thing, but I’m not sure that three would be necessary, especially since it sounds like C and possibly either A or B are already good options and would provide a picture of you from more than just a vet’s perspective.
I hope this helps!