1. the minimum posible throws for this is 5, because if 6 is rolled 4 times you will have filled 24 squares, you need one more to fill the extra square, of course this is a 1 in 7776 chance with (1 over 6) * 5
2. Again the minimum nuber of throws for this is 5, how ever this took me 22, so this minimum, is found out, by having 5 5 times in a row, again a slim chance of this happening
3. i completed this in 17 throws, which shows it is just the luck of the dice, the minimum throws is 5 for the same reason as last time.
conclusion:
the minimum amount of throws for them all is 5.
this is always the case, because 5*5 is 25 and that is the size of the grid, and on the first one, the 6*4 is 24, meaning you still need a 5th row, whereas on the second and third, you cant use the number 6.
but realisticly you would expect the first one to be the least throws and the last to be the most throws