When multiplying hundreds by ones,you have to take it one by one.
For example,on number 5:
199
x 3
1. Multiply 9 by 3,which equals 27.
199
x 3
27
2. Add a zero(to show the next placement) and Multiply 9 by 3,which equals 27.
199
x 3
27
270
3. Add two zeros,and multiply 1 by 3,which equals 3.
199
x 3
27
270
300
4. Now,add it up.
27 + 270 + 300 = 597
Your sum is 597.