I want to take the girls to the local training school.
The next puppy class starts on 24th Sept, the girls will be 17wks.

My problem is I don't drive, and it's a 3 mile walk, which I'm fine with but is it too far for the girls to walk there and back at their age?

Yes definitely too far.

It might be worth seeing if there is anyone else attending who could give you a lift, have a word with the trainers and see if they will pass on your phone number to anyone going your way, I know that happened at my last class.