OK, let's crack this. We're given 391 and our task is to multiply it by 3. Essentially, we'll be adding 391 to itself 3 times.
Step 1: 0 + 391 = 391
Step 2: 391 + 391 = 782
Step 3: 782 + 391 = 1173

So, 391*3 = 1173