Would this work?

REGEX PATTERN (PRCE2 Flavor)(Flags: Global (g))

 (?=.*?(?P<a>A)?.*)(?=.*(?P<b>B).*)?(?=.*(?P<c>C).*)?

Regex demo: https://regex101.com/r/1oYDOF/1

NOTES:

• Added ? to make .* lazy, .*?
• Added ? behind every lookahead group to make them optional.

Great question! I did not know you can capture strings with lookahead. Fantastic!!!! This question opened a whole new world for me in regex. Thank you!