I think what you are looking for is a "compact letter display"

As list of comparisons:

In your case, you don't have enough comparisons.  Should be, e.g.:

library(multcompView)

pairs

Possible. Here you are. The R code without any libraries.

Your results is:

results2

n groups

1 1 P12A,P12B,P12C

2 2 P12E

Example from Salvatore S. Mangiafico

results2

n groups

1 1 P12A,P12B,P12C

2 2 P12E,P12C

JFF

Code Classification Rule