In Korean daily usage, 타당하다, 참이다, 옳다 have same meaning when we specify the thing is right or true.
In philosophy, they are slightly different senses. 타당하다 is the term used in logic, regardless of its truth-value in informal manner, when some sets of statements are valid in logical relation, it is 타당함. So, it has nuance that we cannot verify strictly how much it is true, whatsoever it is it could be plausible.
참이다 is exactly same meaning with being true in English usage. It is affirmative term, when something is right and truthful. The 참인 믿음 is belief that is verified as truthful belief. Often it is followed by illogical but true belief, because of corresponding to disrcepancy in human experience is not bound in logic. However, in philosophy it could be found the way to be coherent discoveries to fit it to logic, modal logic in modern sense researches about this.
옳다 is more value-oriented term, which is similarly to 참이다, but more morally right subject. It is to affirmative expression when someone’s opinion is right, more used in daily usage.
In Korean daily usage, these three terms are interchangible terms, I recommend when you use them, delineate strictly, when you understand other people’s usage, be flexible is ideal way to use these terms. In philosophy, delineative use is recommended.