At worst, it’s a violation of the 3 shevuos which is not one of the 613 Mitzvos, it’s not one of the 13 ikkarim of the Rambam, and it’s not even brought down lehalacha by most poskim.
I agree with you. When did the shalash shevous become such a major thing? The fact that there is so little halachik literature on it before the Vayoel Moshe proves my point. (But still if the three oaths are really halachikly binding, then the punishment for it is terrible, “your flesh will be hunted down like animals in the field” chas vesholom)
Just to quote Rav Moshe Feinstein (Thanks, PAA)
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