You need to use the or function in regexes.

@strings = ('A A C D','A A/B C D','A/B A C D','A/B A/B C D');
foreach $string (@strings) {
  ($third) = $string =~ m/(?:A|A\/B) (?:A|A/B) (\w+) \w+/;
  printf "%s - %s\n", $string, $third
}
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A A C D - C
A A/B C D - C
A/B A C D - C
A/B A/B C D - C
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I put the two alternatives in a group separated by a |, which is the or function. I put them in a group so that it is clear these two alternatives apply only for the first match in the line. The ?: at the beginning means that group should be used for matching purposes but the value should not be captured. I repeat that group because the same two alternatives may show up as the second match.