You have odd integer #1 which I will call (a) and odd integer #2 which I will call (b) where the product of the two (ab) must be decreased by 3, to equal 12.
You add 3 to both sides of the equation... so whereas the 3 cancels itself out on the left hand side and causes the right hand side to become 15. .
Now what two consecutive odd integers equal 15? 3 and 5 do.