Help with Countifs and Conditional Formatting

I have a large dataset with 3 important columns of data (titled Prefix, Mark and Size). Each column contains a mix of text and numeric data.

If there are multiple occurrences where Prefix and Mark values are the same then the Size value should also be the same. I need to be able to identify those lines where the Size value is different (eg those size values in red below).

Please Login or Register  to view this content.

I've been tearing my hair out trying to do something with Countifs and Conditional formatting but am going round in circles.

Any help would be appreciated.

Use Conditional Formatting with a formula, using this formula. This assumes the columns above are A, B, C:

Formula:

Please Login or Register  to view this content.

Only the first occurrence of a given mismatch will be flagged.