Ensure numbers are always three digits long
Ensure first number is always larger than the second
Remove constraint that only need to borrow in the ones place
Remove borrowing restrictions from subtraction 4
Merge pull request #86369 from petercollingridge/sub3

Subtraction 3

Test plan: Ensure that Subtraction 3 uses 3-digit numbers and Subtraction 4 uses 4-digit numbers and that both have variation in borrowing but always have a positive answer.

Auditors: eater, alpert