Hello Cathy1
Thank you for your question.
It is possible that your excel settings have it automatically round to 2 decimal places which is why when you import it, it is automatically changing to the 2 decimal places rather than keeping the 3 when you have exported the file.
The easiest way in Excel to change a decimal amount is on the Home tab, click Increase Decimal or Decrease Decimal to show more or fewer digits after the decimal point.
I would suggest changing this and then importing the data to see if this works.
Please let me know how you go.