At 3 months, it is almost impossible to detect hip dysplasia except for the very worst cases in which the dog would not be as active as normal. I am no specialist in Collies, but puppies often are clumsy things. Did your reading include
www.akc.org? They have good material on all breeds, plus good references. You might find something specific to Collies. Perhaps make the sale conditional on a vet you selected clearing the dog. Unlike me, the vet will be able to see the dog walk. If you have not been using a vet, an early checkup along with transferring the shot records is a good idea. Raising a puppy can be scary at times. Having a vet to call, can be very reassuring. If the breeder did not have a heart worm and flea program you are continuing, you need to talk about them with the vet too. There are good flea products over the counter, but heart worm preventative must come from a vet.
If the puppy is fine, the breeder shouldn't have any problem with the vet check.
Don't buy a big bag of puppy chow. To help reduce the chances of HD, change her over to adult chow in another month.