3) B
We know birds have 2 legs and sheep have 4 legs.
Let number of sheep = S
Let number of Birds = B
Total number of animals = 13 = S + B
Total number of sheep legs = 4S
Total number of bird legs = 2B
Total number of legs = 4S + 2B = 36
Using simultaneous equations,
4S + 2B = 36
[S + B = 13] x 2
4S + 2B = 36
2S + 2B = 26
2S = 10
S = 5