#### Answer

$12$

#### Work Step by Step

The factors of $36$ are $\{1,2,3,4,6,9,12,18,36\}$
The factors of $60$ are $\{1,2,3,4,5,6,10,12,15,20,30,60\}$
The factors of $84$ are $\{1,2,3,4,6,7,12,14,21,28,42,84\}$
Out of these $3$ sets, the greatest common factor is $12$