3
Since I was a little kid I've always counted things by 3's. Or if they're arranged by 3's. The windows in my house are 3 glass panes across and 4 down. So it's 12. 1+2 = 3.
So, I still look at numbers to see if its a 3, or a multiple of 3 (Easy to do, take any number and add it's digits. E.g., 54 is 5+4 = 9. 9 divided by 3 is 3.)
As a CPA, it's easy to spot math errors (transposition errors). E.g., writing down (or inputting) 112 instead of 121. 112 - 121 = 9. 9 is divisible by 3. When the numbers don't reconcile the first thing I do is check to see if the discrepancy is a multiple of 3. If it is, I look for transposition type errors. Usually those are the hardest to spot, but using this 'trick' helps a lot.
Fern