TracFone is now only Verizon coming November. They used to run on all three
US Mobile is still independent so runs on any of the three depending on your phone compatibility. Nice feature is they allow on the fly switching between the three for minimal charge in case you are traveling or vacationing in a location where your current network doesn't have good coverage. Two free switches per month and $2 for each additional switch
My new phone can technically run on any. But it is a Verizon branded phone (supposedly unlocked) but other networks may not accept it. So Verizon is the best bet. I checked with Verizon's compatibility checker and it said it was good. TracFone did too. But my current old phone failed. I have a choice of staying with TracFone and using my new phone. Or switching to US Mobile and using either new or old phone. But at this point, may as well us the new one as it's double the capacity, memory, speed , etc. just need to finish cloning it, copying contacts, texts, photos and everything else