That particular MOSFET, P0903BDL, requires an absolute minimim of 5.0V to work, in fact I wouldn't called it logic-level unless its rated to switch at 4.5V (logic supplies can vary a bit). It certainly not compatible with 3.3V logic.
This is a standard 5V logic-compatible MOSFET - but not rated for 3.3V (although you may be lucky, depending on device-spread)
You apply 12V to the drain, not the source, you're just seeing the body-diode conduct I think.