How to highlight if values in three columns match?

So I work for an insurance company and yesterday they found out that the system is somehow duplicating claims so they have about 10 people going through a spreadsheet to find spots where three columns match/have the same value. I've highlighted the person's ID number in red so we know to ignore the grey values since they're not duplicates so now I'm trying to figure out the best way to identify if the other values match each other.

So I need like an "if column A is equal to column B and column C highlight them or something of that nature." I'm not sure if Excel can do this. Thanks in advance.

If that's not clear I mean if values in all three fields are the same we're going through and marking them but this will take like 80 man hours or more and mistakes will be made.