The greatest common factor of the 3 listed numbers, is 12.
This can be found by simply writing out all the factors of each number.
24's factors are: 1,2,3,4,6,8,12,24
60's factors are: 1,2,3,4,5,6,10,12,15,20,30,60
84's factors are: 1,2,3,4,6,7,12,14,21,42,84
The highest factor shown is 12.
I hope this helps!