compare 2 different numbers and copy the last 3 digits.

Attach a sample workbook. Make sure there is just enough data to make it clear what is needed. Include a BEFORE sheet and an AFTER sheet in the workbook if needed to show the process you're trying to complete or automate. Make sure your desired results are demonstrated, mock them up manually if needed. Remember to desensitize the data.

Click on GO ADVANCED and use the paperclip icon to open the upload window.

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[EDIT]
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Try this in C1, Drag/Fill down

Formula:

=IF(A1<B1,A1&"-"&MOD(B1,10^3),B1&"-"&MOD(A1,10^3))