Answer: Yes, 3 is a common factor of 42 and 84.
We can see this by noticing that
42 = 3*14
84 = 3*28
Also we could use the divisibility by 3 rule which says to add up the digits of the values. So 42 would have 4+2 = 6 which is a multiple of 3. Similarly, 84 has us add 8+4 = 12 which is also a multiple of 3.
Side note: 3 is not the greatest common factor as that would be 42.