So, i want to buy the 3 DLC's for Skyrim, but if i want to do the quests from the dlc's do i have to start the game over, or can i just keep on playing on the safe i got now?

No, the DLCs just need to be installed.

However, because of the nature of the game on the PS3 and how well the devs were able to use the hardware, you might notice some issues depending on how big your gamefile already is. Just remember to do the little things every once in a while to make sure your game runs a bit more smoothly, and manually save often.

Allright, thanks.