To find the sum of an arithmetic series, we can use the formula:

Sn = (n/2)(a + l)

where Sn is the sum of the series, n is the number of terms, a is the first term, and l is the last term.

In this case, the first term (a) is 3 and the last term (l) is 2025. We need to find the number of terms (n).

The formula to find the number of terms in an arithmetic series is:

n = (l - a)/d + 1

where d is the common difference.

In this case, the common difference is 6 - 3 = 3.

n = (2025 - 3)/3 + 1
n = 2022/3 + 1
n = 674 + 1
n = 675

Now we can substitute the values into the sum formula:

Sn = (n/2)(a + l)
Sn = (675/2)(3 + 2025)
Sn = (337.5)(2028)
Sn = 684,660

Therefore, the sum of the series 3 + 6 + 9 + 12 + 15 + ... + 2025 is 684,660.