The formula that I provided for you referenced row 4, I don't understand why you changed it to row 3. This is one reason why it failed.
The other reason is that you did not change ; to \

You shouldn't need to change this one
Formula: copy to clipboard 
=IFERROR(1/(1/MAX(FREQUENCY(IF(C4:LT4={"R"\"M"\"I"};COLUMN(C4:LT4));IF(C4:LT4<>"R";IF(C4:LT4<>"M";IF(C4:LT4<>"I";COLUMN(C4:LT4)))))));"")