Stuck at 3 million border mark minecraft
Solution 1:
You will need to get the server owner to either use a program to move you back into the playable area, or delete your player file so you spawn back at the world spawnpoint.
MCEdit is a third party program capable of opening world files and moving players.
For the deletion method, your player data will be stored as a file with the name as your UUID in the "playerdata" folder of the world file. Note that deleting this will also remove your inventory (I'm guessing you're using commands and or creative mode anyway if you got past the 30 million mark).