Would this powersupply work to drive 3 xm-l:s in series in a modded worklight?

12v 3a power supply

Do you still need to connect a driver between the PS and the leds, if so, can anyone recommend one?

Output voltage is adjustable, so limiting the voltage will also limit the current through the LEDs. BUT, not knowing the range of adjustment, it's impossible to say. You would need to get it down to around 9.6-10v for 3 XMLs in series, depending on what current you wanted to run them at.