**Neither a teacher nor a native speaker.**
We were there for 3 days and had some/a reason [STRIKE]so[/STRIKE] for that.
In my opinion "a reason" sounds better than "some reason", but both should be correct.
emsr2d2 has already mentioned that it's not a complete sentence.
Now (with my changes) it's a complete sentence, however, I don't know what you actually want to say.
Cheers!