I was awaken once by 3 loud knocks on my door. I looked outside and not so much as a print in the snow. I know this is different than your experience,
but someone said they had read that Jung or someone had mentioned the importance of the number three and knocks; I found no info on it myself,
though

. A friend had a simalar experience at roughly the same time.
Anyways, I wil keep my eye on this thread to see what develops, so be sure to let us know!