To round off an answer to 3 significant figures look at the fourth figure after the decimal point.
If C is greater than or equal to 5, increase the third digit after the decimal point by 1 while eliminating C, else eliminate C without making a change in the third digit after the decimal point.
As an illustration: 23.0145 would be rounded off to 23.015 while 23.0144 would be rounded off to 23.014
Once this is done, the number has been rounded off to 3 significant figures.
any number other than 0 is a significant number, so when you round you should have 3 numbers that are not zero and other zeros but make sure the 0s are behind the 3 number. i.e:
1536 rounded to 3 sig figs is 1540
363852 = 364000
1.3560 = 1.36