[Madd92]

Create a variable for every user and save the unix timestamp (gettime() + 3 * 60) when the command is executed and test if when it is executed again, the saved timestamp is bigger than the one gettime() returns then. If it is, give them a message or what ever that he has to wait.

Edit: please don't use a timer, it's not necessary and takes up a lot of performance for many users/commands.