Add 2 and 3 vectors Geometrically
To add 3 vectors add A and B to create the red resultant A+B. Now add this resultant to C (or add C to A+B) to create the red resultant A+B+C.
Hide everything except A, B, C and the resultant A + B + C. The vectors are in equilibrium if the resultant is the zero vector i.e. disappears. Drag point C until the 3 vectors are in equilibrium. You may need to use the Panning Tool to move the screen around!