How to add repetition of the same letter combination on a mask?

Hello, I would like to generate passwords that follow this pattern
<3 capital letters> <same 3 letters undercase> <same 3 letters undercase> <same 3 letters undercase>_9

which would generate (skipping AAAaaa... and other repetitive permutations)
ABCabcabcabc_9
ABDabdabdabd_9
...

So always the same 3 letters repeated four times (those 3 letters all possible combinations of the alphabet, so from AAAaaaaaaaaa_1 to ZZZzzzzzzzzz_1 and everything in between).

I'm thinking the repetition should decrease the amount of iterations that would be needed to find the match.

I have read the docs on the wiki, but could find how would I reflect this repetition in a mask for hashcat (rule based attack seems the closest thing to it, but I can't still wrap my head around it).

If you could point me in the right direction with an example, I'd appreciate it. Thanks.