Is the following solution right:
1 step - Check if all the nodes in the graph have exactly 2 edges enter\exit them. If not, either there is an edge that not part of a circuit, either there is an edge visited more than twice (within it's own circuit or an other circuit). So, if not, just reject.
2 step - Apply IS (G, 3). If there is an independent set of size 3, we have 3 different circuits.