how do i find the Fnet of 3 planets? the 3 planets are also at angled distance from one another(2d).

Planets attract each other in pairs. Figure out the three pairs of forces.

Now, you Fnet of all three will be zero (each of the three pairs are equal and opposite).
If you mean Fnet on one of the planets, then you have two pairs of planets. Figure each force on the planet of interest, then add them as VECTORs, to get the resultant.