Excel: Round or truncate to 3 decimal places

Hello,

I have a small problem with Excel regarding decimal numbers.
Actually, I have a column of prices where the numbers can have 3 or 5 digits after the decimal point. When I format the column as a number, I can enter all the prices with 3 digits after the decimal point. This changes the visual part, but when I select a cell, the content hasn’t changed... anyway!

Is there a combination of TRUNC and ROUND formulas that would allow me to achieve the simplest possible result:
- round these values to 3 digits after the decimal point if the next digit is greater than 5
- truncate to the 3rd digit if the next digit is less than 5.
For example: 3.50265 would give 3.503 and 3.50245 would give 3.502

I absolutely need to have this result in order to compare the values in this column with values from a reference that is also at 3 digits after the decimal point.

I hope I have been clear...
Thank you very much in advance for your help?!