Like Yokes said you will get a key for steam/Gog a year later as well as a key for the final game on Epic in addition to the backer build key.
You are free to do what you want with the keys.
If you redeem the backer build key you will need to close the Epic launcher in the system tray and reopen it to clear a bug that prevents your library from updating with the backer build
:edit: since you are one of the people that pre-ordered before March 12th, your copy of the game on epic also includes the dlc. You are going to want to read this thread to know everything you are getting