Can I use an at&t or sprint iPhone 6s for T-Mobile?

I was looking into buying a new phone and on the Apple website the at&t and Sprint iphones were only $200 and the T-Mobile iPhone was $650

If you get it unlocked. If it's locked to a carrier it won't work with any other. The reason it's so much cheaper is because you will have a contract.

That's $200 down snowflake, and the rest you pay in an installment plan.

It ain't what it seems my little friend. You pay the same price for an iPhone, one way or another, regardless of where you buy. It's all in the contract details. You should realize you can't get a phone for $200 and take it to another carrier

Like everyone is saying you can pay $200 down and have to pay that phone company full price like over $700 over the next two years.
I paid over $800 for wife's Unlocked 64 gig iPhone 6 with apple care accent plan and can use it with Cricket, AT&T, or T-Mobile