three months... into the future
Months is spelled months, not mounths.
In this quote, they are talking about their wedding that is going to happen in three months. It is three months into the future.
If they were talking about three months ago, it would say, "three months behind."
The definition of ahead can mean in the future:
- in, into, or for the future
Another way to say your excerpt with the same meaning:
The captain put his arms around the girl’s shoulders. They talked about their wedding, to happen in three months. He had not, after all, ever broken their engagement.
The captain and the girl are engaged and never stopped being engaged (never broke their engagement). They will get married in three months. Now they are talking about their upcoming wedding.