If you right-click on your numeric field and choose format, Customize..under the number tab you get a bunch of choices to set the decimal separator, how many decimals to display, etc. choosing the formula button next to Decimals, you could put in a formula that looks at the amount of digits after the decimal dot and then either set the decimals to 2 if there are more than 3 decimals, and to zero decimals if there are 2 decimals. They key here is to get the order right. You want to check for the 2 decimals first and then for 3 or more decimals. If you do it the other way around the formula will display everything without decimals.
Hope that helps,
Chris