Yes, your eeros are interchangeable. During setup, you can designate any eero as your gateway eero. The gateway eero will be the first eero that you connect to your modem and the one your other eeros use to reach the Internet.
If for any reason, your gateway eero goes down, you can replace it with any of your other eeros. Simply replace the faulty eero with another, power cycle your modem, and you should be good to go!