The iPhone 6, unusually amongst smartphones, has such a wide variety of radio bands, it can roam, or be used directly, on virtually any network in the US, (or the world!) and obtain 4G LTE service on any network. Which network actually gets used will depend on the SIM you are using and the roaming agreements that that provider has with US carriers.
But the large selection of radio bands does give the option of using a US based SIM card, if you so desire, from almost any carrier (except Verizon may not work or be limited to 4G only).
The only thing you will need to do is to unlock the phone (if it isn't already unlocked). EE will do this for you if you don't have an outstanding balance and also:
Your device must have been purchased when joining your contract or upgrading and actively used with your pay monthly SIM for a minimum of six months.
There does not appear to be a cost for unlocking the phone.
Once you receive the SMS stating that your phone has been unlocked, you should be able to use any other SIM card in the phone.
Finally, note that with Three, your data usage in the US comes out of your add-on allowance, so you will want to have one of the add-ons with unlimited data. You'll also need to activate roaming on your account.