Just state that you will send the extra refs at a later date. From what I can tell from talking to people at vet school and interviews etc, they will be able to tell if you have actually got experience from the questions they ask you, and what you have to say. I have spoken to people who didnt send any refs in and still got a place at vet school.
I shall be sending the refs I have with my extra ps, and the extras as I get them.
I'm pretty sure I read the extra ps had to be in by 3rd December so Id say you have a similar time frame for refs, though id guess the earlier you can get them in the better.