Today was fairly benign at three: Norway > Latvia > Germany (Thank you Air Baltic!).

I think my top is four, although technically it took place over more than one calendar day: Afghanistan > Turkmenistan > Germany > USA.

Other three country excursions:

Jordan > UK > Hong Kong
Singapore > Japan > USA (more than a day, but crossed int'l date line so it was actually one day )
USA > France > Germany
Hong Kong > Netherlands > Germany

Actually, now that I think about it: Germany > UK > Germany > India.

Anybody got any real crazy, multi-country in-a-day itineraries?