could not understand why it is partial constraints. Also, I don't understand how link works. Below here is what I am thinking i think it is totally wrong can someone explain what's wrong with my idea here.
The blue rectangle is not fully constrained by the three links within the frame. The position of A is fixed by the first two links, but the diagonal link 3 may be stretched as the body is rotated slightly about A.
The top of the cosine function at zero, is horizontal. Because the line from A to link 3 is straight, very high tension forces could be present in link 3 if the body was rotated slightly.
As @Baluncore stated, the tension on link 3 goes to infinity when its attachment points are co-linear with point A. The situation is similar to a rope or chain stretched tight between two supports. Try calculating the tension when pushing on the center of the rope/chain span.
A practical use of the phenomena is retrieving a vehicle from a ditch.
Two people with a chain, a tree, and something to block the wheels, can pull the vehicle out. Providing there is a portion of the chain not being supported (by the ground):
chain taught between tree and vehicle
person 1 stands on the middle of the chain, deflecting it
person 2 chocks the vehicle wheels so can't roll back down into the ditch
person 1 steps off the chain and repeats from step (1)