why there is 3 hydrogen bonds between guanine and cytocine and 2hydrogen bonds between adenine and thymine

The guanine forms three hydrogen bonds with the cytosine because In cytosine, the amino group acts as the hydrogen bond donor and the C-2 carbonyl and the N-3 amine as the hydrogen-bond acceptors and in Guanine the group at C-6 acts as the hydrogen bond acceptor, while the group at N-1 and the amino group at C-2 act as the hydrogen bond donors. 

The adenine forms two hydrogen bonds with the thymine as only group at N-3 and carbonyl group at C-4 in Thymine form the hydrogen bond with the amine group at C-6 and group at N-1 of the adenine.