To round a three digit number:
Say we have 678. We look at the third number which is 8 and round up because it is greater than five. When we round up the seven turns into an 8 and now we have 680(to the nearest ten). If you want to round a three digit decimal like 0.789 do the exact same: 9 goes up to 0 and 8 goes up to 9 and now we have 0.790 to the nearest ten.
To the nearest hundred look at the second number and round
Hope I helped!
