Milbemax usage is vet trying to get me to buy more then needed?

I had my 7 week old kitten in the vet today, the vet gave me milbemax and told me to do one dose now, one in two weeks then an additional dose in two weeks. He said this was the normal routine. Ive had 5 other kittens dewormed and the other vets told me a dose now and one in two weeks was fine. He doesnt have tapeworm segments near his bum, normal poops. Just a bit of a pot belly. I really dont want to give the third dose if its not needed?

Well, from what I've read, sometimes the treatment is at 6, 8, 10, and 12 weeks, so you might be getting of easy.  And to prevent heartworm, then give it once every 3 months thereafter. 

Previously, were you treating your kittens with the same drug, or something different.  Naturally, each drug is different.