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?

I did four on the same calendar day:

STR-ZRH-WAW-EWR-CLT-MSP
Germany-Switzerland-Poland-USA-USA-USA

Going eastbound does make it easier. My first cited trip (Afghanistan-US) started in the early evening and ended mid-morning the next day.

Were you actually in those countries or just connecting in airports that happen to be located there? The latter doesn't count IMO.

Getting three wasn't too hard with StarMegaDo as we had parties in Germany, Norway and France on the same day and that was more than just airport connections. Also possible was USA-Panama-Mexico on the recent CM H-fare sale. I did that twice earlier this year, visiting the Panama Canal both times before heading on to Mexico City.

My other 3-country day started in the USA then went to the UK overnight and Luxembourg later than night. All in 24 hours but had the overnight of the red-eye to miss the calendar-day requirement.

For anyone who transfers through Europe to fly to a third country three or even four countries is routine and pedestrian............

three in a day a few times
Germany/France/Switzerland
Qatar/UAE/Bahrain
Spain/Portugal/Gibraltar
Poland/Czech Rep/Germany

four
Poland/Slovakia/Hungary/Netherlands



FIVE within 24 hours, but on 2 calendar dates on the island hopper to Bali

Hawaii(USA)/date change/Marshall Islands-Micronesia-Guam-Indonesia!

Probably a better metric to use is from head off the pillow until head hits the pillow again (in an actual bed).

But are you counting simple connections or does one have to actually do something? In most of the world - other than the USA, Canada & the UK - international transit zones make it quite easy to be in a country and not really there at all at the same time.

Years ago I took a bus from London to Italy - from memory the route took in UK, France, Belgium, Luxembourg, Switzerland and Italy (six).

Plenty of bus routes across Europe would take in more.

Four:

South Korea - Macao - Hong Kong - Singapore
Croatia - Montenegro - Serbia - Kosovo (would have been five but couldn't find the turning to Bosnia)

Five using the same countries more than once:

UK - France - Belgium - France - UK

I did Tbilisi to Budapest by bus with no stops other than changing buses in Istanbul and Bucharest. (Georgia, Turkey, Bulgaria, Romania, Hungary).

The bus trip actually started in Tehran, but I spent a week in Georgia before doing the long road to Budapest. (The best bus was the Tehran-Erzurum segment).

My only fiver:
Portugal, Netherlands, France, Canada, USA (flew LIS-AMS-CDG-YUL-EWR)

Driving, which gets around sbm12's rule that "staying in airports doesn't count", I have four-tricks:
Switzerland, France, Andorra, Spain
Italy, Switzerland, Liechtenstein, Austria

Fairly easy to do in that part of the world.

If repeating countries can count, I might have 10+. I sometimes stay with a friend who lives in Switzerland, very near the French border. We get up in the morning and go get the breakfast bread from the boulangerie in France then come back home to eat breakfast. Then on the way to somewhere in Switzerland, we might take the road through France because it's faster. Rinse and repeat in the afternoon. Then go for meals in France because it's both cheaper and better than Switzerland. We could easily do CH-FR-CH-FR-CH-FR-CH-FR-CH-FR-CH in one day. I never kept track but I am sure I have done it 10+.

and I once upon a time with a friend jumped back and forth across the border between Spain and Portugal at least 200 times at a closed border post in the 80's......

Not sure if this is my best, but first that came to mind.

4 countries:

Woke up in Monaco, caught a flight out of NCE (France), connecting in Zurich (Switz) and landing in Athens (Greece).

KUL-SIN-HKG-LAX

Malaysia- Singapore - Hong Kong, SAR - USA