Look at the title page. You'll see a string of numbers, like 9 8 7 6 5 4 3 2 1. For every printing, they delete one number, so a 3rd printing would be 9 8 7 6 5 4 3. Basically, the lowest number in the sequence is the print run for that particular book.
I am pretty sure only the PHB is into a 6th printing. At least that is the only one they officially announced. I think the other two are still around the 3rd or 4th print range.