What is is 3 15 what equal 12?

The expression "3 15 what equal 12" seems to be a bit unclear. If you're asking for a mathematical equation, it could possibly refer to finding a number that, when combined with 3 and 15 in some way, equals 12. For example, if you subtract both 3 and 15 from a number, you would need to solve for that number. If you clarify the question, I can provide a more precise answer!