If you subtract 3 as 8, the result is 8, then subtracting 8 is 8, and this number is 8+8 = 16.
The correct result should be 16- 3 = 13.
Of course, it can also be understood that subtracting 3 means subtracting 8, and subtracting 8-3 =5. The result will be 5 less than the original, so the positive result is 8+ 5 = 13.
See, honey?