(a) Since the body is moving with no acceleration, the sum of the forces is zero =F1 + F2 + F3 = 0. Let F1, F2, F3 be the three forces passing through a point. Let F1 and F2 be in the plane A (one can always draw a plane having two intersecting lines such that the two lines lie on the plane). Then F1 + F2 must be in the plane A. Since F = - (F1 + F2), F3 F is also in the plane A.
(b) Consider the torque of the forces about P. Since all the forces pass through P, the torque is zero. Now consider torque about another point 0. Then torque about 0 is
Torque = OP x (F1 + F2 + F3)
Since F1 + F2 + F3