How to combine target selector arguments with OR, not AND?
You cannot do that. You have two options:
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Have one command per possibility.
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Invert the check and the arguments (not always applicable):
/execute unless @s[tag=!a,tag=!b,tag=!c] run […]
That command runs if you have either tag a or tag b or tag c, because it says "do not run this if I do not have a and do not have b and do not have c, otherwise run it".