if you start your instances with this parameter set, no matter how many instances you have, you use rs.initiate() only once on one of them and then use rs.add() to add other members.
you could also use a configuration on initialization if you knew what would their IP addresses be. it is a tedious thing to set up but not impossible (play on compose file):
rs.initiate({
_id: "rs0", members: [
{ _id : 0, host : "mongo1:27017"},
{ _id : 1, host : "mongo2:27017"},
{ _id : 2, host : "mongo3:27017"}
] } )