This question seems difficult but when you start knowing what it means, things become easier.
When you say pot odds are 3:1 to call it means that in this situation you only need to win 1 time out of 4. Let's put on example, this pot odds occur when villain bets half pot:
Let's say on the river pot is 2€ and villain bets 1€. When you lose on showdown, you lose 1€ (your call), but when you win, you earn 3€ (his bet and the pot).
- 1# lose 1€
- 2# lose 1€
- 3# lose 1€
- 4# win 3€
So, what is happening? In this situation no matter what if you win 25% of the times, the EV will be 0.
Thus, the answer of your question is, if he is bluffing more than 25% of the times you can call him with all bluffcatchers
Always try to think what are meaning the pot odds and you will think for your own