GA4 numbers are 30% off from what my ad platforms are reporting, which one do I trust

Meta Ads Manager says one number, Google Ads says another, GA4 says a third, and none of them are close. Client wants a single number for the board deck. I know attribution windows are different but a 30% gap feels too big to just be that. Is there a standard way people reconcile this or do you just pick one source and defend it?