Does area effect stack when say i play a gun drones on a tactical squad cardinis giving my a unit with area effect (3) or do i simply have a unit with area effect (1) and area effect (2)?

As per the RRG, it stacks. Any keyword which uses a number (such as area effect), the numbers accumulate if you gain that keyword again. Your example unit would have area effect (3).

However if the keyword does NOT use a number, you get no benefit for having the keyword twice (so you can't be double-brutal, for example).

RRG, p9, subheading "keywords".