Thoughts on the 3x case
Since George is moving at 3 times the speed, the possible locations to meet will be a diagonal parallel to the diagonal where they meet but close to B. For this reason, there will be fewer possible points of collision and you would expect the probability to go up.
BUT! Since George moves 3 spaces for every space that Lennie moves, there is a good chance that George will run past Lennie in a single move.
I suspect that there are certain values of N where no collision is possible - but I haven't worked through the details.