To find the sum of the first N terms of a geometric sequence, we can use the formula:

Sn = A * (1 - r^N) / (1 - r)

where Sn is the sum of the first N terms, A is the first term, r is the common ratio, and N is the number of terms.

In this case, A = 3, r = 2, and we want to find the sum of the first N terms.

Sn = 3 * (1 - 2^N) / (1 - 2)

Simplifying the formula:

Sn = 3 * (1 - 2^N) / (-1)

Sn = -3 * (1 - 2^N)

Therefore, the sum of the first N terms of the given geometric sequence is -3 * (1 - 2^N).