How to house train three puppies with fake grass potty pads indoors?

Well, you have one heck of an undertaking to try to train 3 puppies at once. Housetraining a puppy is not as easy as you may think, and there are many things that can affect how easily a puppy is housetrained. Unfortunately, training multiple puppies at once and training to an indoor surface are all particular challenges with housetraining a puppy.
I am attaching a link to a handout below with very detailed information about housetraining.
https://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=2421
And this is an article on housetraining pitfalls.
https://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=2461
I will say that it is often easier to train puppies to a fake grass surfaces outside, then move that surface inside. That will train the puppies not to urinate or defecate anywhere else inside the house which can be a huge challenge with smaller dogs. So, my best suggestion would be to take the puppies outside to urinate and defecate, but to place the fake grass outside so they associate that with an appropriate area to urinate and defecate. Only once they are complely house trained would I start bringing the fake grass into the house (which will likely be a couple of months). If you do it too quickly, you will risk them always going in the house in inappropriate places. If taking them outside isn't an option for whatever reason, the only suggestion I would have is to confine them to an area that only contains fake grass (using a kennel or an X-Pen instead of the entire room). Again, the idea is that they never go anywhere except on the grass, so they won't have accidents in other areas of the house.
Good luck! I hope that was helpful and gives you some ideas. Thanks for using PetCoach.