How to Give unique user id when many users registered at same time

Question

Hi!

i got a serious problem and problem is....

I've three different tables for different user types with same column Uid, when user click register button i'm just checking biggest of three Numbers from all three tables and inserting one greater than the biggest one...


Now when more users register simultaneously on server side, it is inserting same Uid for all users registered at that moment...

How should i achieve unique id for all users registered at that moment..

is there any solution??? Uid is integer....

Please Help me... I'm strucked very badly so please...