There are several different ways of doing this. For your purposes, the simplest way might just be to do an extra check after spawning to see if any player objects are overlapping, and if they are, to reassign them to a new square. You'll want to do this in a While loop to ensure you don't randomly put them into a different occupied cell. (This isn't a great idea if you have a very large number of player sprites, but for just 3, this should work just fine).
Eg. On start of layout:
-------Spawn Player 1 at random coordinates
-------Spawn Player 2 at random coords
-------Spawn Player 3 at random coords
--------------WHILE Player 1 is overlapping Player 2 OR Player 3
---------------------Set Player 1 position to random coords
--------------WHILE Player 2 is overlapping Player 1 OR Player 3
---------------------Set Player 2 position to random coords