in front, 50% chance (but probably less than 50 in a simulation as the toad in front didn't croak male (Since it hasn't croaked yet).
behind assuming equal choices of your case (2/3)*0.5 + 1/3*(1) = 2/3
I wonder if there is some chance that because the back group of toads croaked first (in a simulation) that there's an increased probability that it's the 2 male toad group vs. one of the other two. That's maybe going a little too deep.
I'd drag my feet a little on licking the left toad of the double group just in case the other one croaks, though.