Let's suppose that the pot odds is X:1 ( The pot is not X dollars, and you have to call 1 dollar )
The formula that you can use to take a decision on calling or not is this :
Win odds * (x+1 ) > 1 : the call is a profitable action, other way that will be unprofitable.
In your example the pot odds is 3:1, that means 33% *(3+1) =132% > 1 ====> profitable action.
The pot adds reflect the ratio of the pot for one dollar that you put now, that means , example : the pot now is 10 $, you have to put 2$ , so the pot odds is 10:2 or 5:1, but to calculate the profability of the action we have to take the size off the pots after the call not before , let's say here the win odds is 19% , then the global ratio will be 19%*(5+1 )=114% .
I hope it's clear for you .