I have a Gs3 on verizon i want to know if i buy a sim card from sprint can i use the phone on sprint?
Welcome to the forum
I strongly suspect not. The phone would have had to be an unlocked phone toi be able to use it on any provider. Every provider has their own (special) firmware
I have a friend who took an HTC One that was on ATT, and put it on Verizon using the Verizon Sim card he had in his iPhone. He called Verizon support on how to do this and they would not tell him how, but gave him hints on where to look to get the information. They also said that they do not "lock" phones any longer, meaning that it can't be moved to another carrier.
I'll PM you the instructions when I talk to him again, but I think the answer to your question is "yes," you just need to reprogram the phone.