apparently 26x26x26 = 17,576 would consider rare but then you can reduce that figure; c d n are kinda normal letter combination, e.g. you not gonna get any people with z z z or z x y or z x v etc but you will oftener get initials like say j a s possibly john anthony smith or jane ann spencer So you got quite a lot of figuring out to do.. my guess is about 1000 to 1 unless anyone knows better?
J, P and A are common initials for first names; S, M and B are common in surnames, so I think a 1000 to 1 is a bit high-particularly if neither of you have a middle name.