AST

Tenancy Types and Management

Hi, I have a student property which would normally be marketed for Letting pretty soon, however 2 of my 3 students are keen to stay in the house as they have 1 more year to go on their courses, the third is waiting to see if she gets on a PGCE course, she won't know if she's got the place until the end of January.
I would like to sign up the 2 students who say they're staying as soon as possible, could I do that with an AST and then modify it later to fit in a third tenant or would I have to create a new AST when the third student confirms or I find another student to fill the house if she doesn't get her PGCE place?

regards