Even if what Gaspar says is true in game, it is still not a very good way of limiting party members. First of all, all of your party members leave the end of time at the end of the game, proving that it is possible for a group of 7 to leave the end of time. Secondly, since all of the gates go to the end of time, it doesn't matter if more than three people enter a gate, since they would be going to the End of Time anyway. So it is possible for a group of 7 to enter and leave the end of time, and, if this wasn't a game, Crono could have brought his full team wherever he wanted.