3.a)
The force 
is a normal force. It is exerted by the plane on
the block. The force 
is a
gravitational force. It is exerted by the earth on the block. The force
is an applied force which is exerted by the hand on the
block. The force 
is a frictional force. It is exerted
by the plane on the block.
b)
The Normal force is balanced by the components of and
in the direction of . The Frictional force,
is balanced by the other components of the weight and
the applied force.
c)
The magnitude of the net force on the block is the same as in part a,
and is identically zero if the block is at rest. The magnitude of the
frictional force is less than in part a, because in part a, there was a
component of the applied force which was opposing the frictional
force, thereby increasing the value of necessary for the
block to remain stationary.