It's simple consider the states as remainders when some binary number is divided by 3.i.e.y mod 3==0.Thats why q0 is final state as the remainder will be 0 there only.
For example if we take ,1001 = 9
9mod 3 =0
And now check the transitions from initial state.
q0 on 1 goes to q1
q1 on 0 goes to q2
q2 on 0 goes to q1
q1 on 1 goes to q0.as it reached to final state .so this string is accepted .hope you got it ...