Final answer:
To solve 3 + ___ = 15, subtract 3 from 15 which equals 12, so the complete sentence is 3 + 12 = 15.
Step-by-step explanation:
To complete the division sentence 3 + ___ = 15, you need to determine what number, when added to 3, equals 15. This is a simple addition problem, not a division problem, despite the mention of division in the question. By subtracting 3 from 15, we find that the missing number is 12. Therefore, the complete sentence is 3 + 12 = 15.