To find the value of C, we need to sum the geometric series 3 + 3^2 + 3^3 + ... + 3^1000.

We can use the formula for the sum of a geometric series:

S = a * (r^n - 1) / (r - 1)

where S is the sum of the series, a is the first term, r is the common ratio, and n is the number of terms.

In this case, a = 3, r = 3, and n = 1000.

Plugging these values into the formula, we get:

C = 3 * (3^1000 - 1) / (3 - 1)

Simplifying further:

C = 3 * (3^1000 - 1) / 2

This is the value of C. However, it is an extremely large number and not easily calculable.