Server merge combines multiple servers into a single server. Your old character will remain in the same server you registered on.

If you played on S2, and it was merged with S1 and S3, then you would need to enter the game through S2 to play your old character. If you enter S1 or S3, you will still end up in the same server, but you will create a new character.

To find the server number that you have played before, you can visit the game's recharge center.