A sequence follows an exponential pattern when each successive number increases (or decreases) by the same percent.
Then, we can define the intial value as 3 and we want the common ratio to be 2, meaning that each term doubles its previous value:

We know it follows an exponential pattern because the ratio between consecutive terms has a constant value:

for any value of n.
Answer:
3, 6, 12, 24 are four terms of an exponential pattern where each term is the double of the previous one.
As the ratio between consecutive terms is constant, we know we have a exponential sequence or pattern.