Attack mode 6 only uses 1 GPU.

[undeath]

You're not providing enough work to hashcat. Hashcat's performance highly depends on the left part of the attack. In a6 that means the wordlist. As a workaround you can either pipe in the words:

Code:

hashcat  -a 6 wordlist.txt ?a?a?a --stdout | hashcat -m 18300 hash.txt -w 3 -o output.txt -D 2

or use append rules with -a0 -S instead of -a6.