Based on these web sites, MC beats Visa for EUR and loses to Visa for GBP.
Visa tool:
https://usa.visa.com/support/consume...lculator.html/
MC tool:
https://www.mastercard.com/global/currencyconversion/
Based on these, Visa is usually about 1% more expensive for EUR charges paid in USD, with a range of about 0.75% to 1.25% higher. Charges in GBP charges however, were cheaper for Visa, by comparable amounts.
I can't find a similar tool for Amex, the closest is a Barclays UK which is only GBP
https://www.barclaycard.co.uk/person...rds/amex-rates. GBP charges were cheaper than Visa or Amex, but I'm not sure how accurate this is, as I paid almost 1.5% more using an Amex SPG card.
Is there a general Amex currency conversion tool?
You might see if the reported rates equal what you were actually charged.