OK, let's work through this together. We're starting with 3 and we're going to multiply it by 7, which means we add 3 to itself 7 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

So, 3*7 = 21