Answer: 3, 6, 12, 24
Step-by-step explanation
The nth term formula for this geometric sequence is

Let's plug in n = 1

This helps confirm the first term is 3. It also helps confirm we have the correct geometric formula.
Now try n = 2.

The second term is 6. A shortcut would be to double the previous term (3) to get 6.
If we double the second term, then we get the third term is 6*2 = 12
Notice how:

Then finally use the formula or the shortcut to determine the 4th term is 24.
In summary, the first four terms are: 3, 6, 12, 24