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@tage
to find p-channel MOSFETs that operate with 2.5V on the gate you could look at this page
http://www.mouser.com/catalog/catalogusd/646/674.pdf and check out the part numbers that are listed with Rdson at 2.5V.
if you want to connect the p-ch MOSFET to +12V you will of course need a part that has high enough gate voltage rating. so you may choose one that has Rdson rated only at 10V and 4.5V, and 20V gate voltage max rating. some of the parts that need very low gate voltage also have very low maximum gate-source voltage rating, for example 8V maximum.
there are different ways you can drive a p-ch MOSFET connected to +12V from a logic signal with 3V amplitude. you could use a npn transistor with base connected thru a 10k resistor to 3V and the emitter to GPIO pin. the collector connects to the gate of the p-ch MOSFET. the source of the p-ch connects to +12V and you need a resistor (10k) between gate and source, to keep the p-ch off when he npn is off.