Upon issue, the visa is supposed to be good for 6 months. I have heard of a few cases where the consulate apparently sat on it after issue, and didn't deliver it to the recipient for a while. But the normal is: it's good for 6 months from issue. This means (normally) that you have a 6 month window in which to travel to the U.S.
The 3-month-period is the window of time during which you have to marry AFTER arrival in the U.S. (or else the K-1 becomes invalid.)
Congrats again !

-MK