Subtract letters in a formula

I am attempting to find a way to "subtract" letters from a string of letters. In the workbook I've attached, I have the example show that I'm looking to achieve.

Basically, in cell A1 I have "CCT" and in Cell B1 there is a "C", in C1 there is another "C" and in cell D1 there is a "T". What I would like to happen in A1 is to remove a C or the T when one of the characters is entered to any of the cells. Essentially any combination should remove the equivalent. If CCT is entered into B1 then a "-" or blank should be present.

Col E was my bad attempt at trying to subtract it. I know how to concatenate but not how to remove it. A1 is representative of what would start out in that column. In retrospect I should have put a “-“ there instead.