Let's roll up our sleeves and solve this. We have 3 and we're going to multiply it by 15, essentially adding 3 to itself 15 times.
Step 1: 0 + 3 = 3
Step 2: 3 + 3 = 6
Step 3: 6 + 3 = 9
Step 4: 9 + 3 = 12
Step 5: 12 + 3 = 15
Step 6: 15 + 3 = 18
Step 7: 18 + 3 = 21
Step 8: 21 + 3 = 24
Step 9: 24 + 3 = 27
Step 10: 27 + 3 = 30
Step 11: 30 + 3 = 33
Step 12: 33 + 3 = 36
Step 13: 36 + 3 = 39
Step 14: 39 + 3 = 42
Step 15: 42 + 3 = 45

So, 3*15 = 45