The server merge will come like this.

1. Players will have the luxury of character selection during logged in.
So if you have characters in S1/S2/S3 when you logged in you will be able to select which character to play with.

2. Groups that have similar name, a temporary group name will be given to all. With this all group leader will be given a Group Rename token to update your group names ^^

3. Yes the server merge will be for those earlier server / based on populations of the server itself.

Regards.