June, July, August sabbatical planning recommendations?

planning on spending the entire months of June, July, and August in Europe

You are aware of the 90 day limit?
June, July & August total 92 days

From a weather standpoint I would probably do Greece 1st and France last, to avoid the worst heat, but plan on being hot in July and August at any/all of your choices

I recommend staying in Greece in June before the heat gets bad. Avoid Greece in August when locals are on holiday.

June, July and August... I have one word of advice for you:

NORTH

Much of Southern Europe is often miserably hot in July and August. Southern Spain in July, especially in August? No thank you! Same for southern Portugal, all of Greece. Northern Spain, OK. Coastal Portugal, maybe. Greece - in August? Yikes.

You say you want to avoid the summer heat. If that's true, I think you will want to re-examine your list of countries. The places you list are all wonderful...except in mid-summer, when the heat can be absolutely unbearable (crowds, too). You can usually get away with decent weather in June, but in July and August, I'd stick to northern Europe as much as possible (it's an ideal time to visit Ireland, Scotland, Scandinavia), where the worst of the summer heat should not bother you much.

and the 90 day limit includes arrival and departure days, any part of a day in the Schengen zone.

Thank you all for your excellent suggestions! I did not know about the 90 day limit.

You may find this helpful

https://www.schengenvisainfo.com/visa-calculator/

With the 90 day limit, you may also want to give yourself some wriggle room in case you test positive for Covid at the end of the trip and can’t board your flight home.

You have opted for southern Europe, yet you say you want to avoid the heat.

Greece will be hot in June, but bearable. I wouldn’t go there in July or August. Crete has plenty to keep you occupied.

France - avoid the south of France in July and August. Normandy and Brittany are more temperate. Accommodation prices rocket (triple) around 5 July when the French go on holiday for 6 weeks.

Spain - northern Spain would be a good part of Spain to visit in July or August as it’s also more temperate.

Portugal - avoid the Algarve and inland parts, which will be blisteringly hot. I have had 45 degrees in the inland Alentejo in early July. The area between Lisbon and Porto will be slightly cooler than the Algarve, but still hot. The coast is obviously going to be busy as it’s school holidays.

If you really don’t want heat, then you would be better looking at northern Europe or visiting May to July.

And....when you are pulling out your calendars, give yourselves some flex days at the end. IF the US is still requiring a negative Covid test to fly you may be in your last country for 10-12 more days if you happen to get a positive test result. You'd need that flex time if that happens so you don't overstay your allowable time. I have read on the FRance government site they are allowing people to extend time if it is due to Covid that they overstay but I'd not want to chance that rules might change while I was traveling.

You didn't mention Great Britain in your list of interests but it is not in the Schengen travel zone so has differing stay requirements for US passport holders. You can certainly combine 75 days or so in the Schengen zone with up to 6 months in Great Britain.

Schengen countries:
https://en.wikipedia.org/wiki/Schengen_Area

UK visitor rules (no visa needed at this point for US passport holders)
https://www.gov.uk/standard-visitor

Do you know where you want to go in each country?

Are you beach people, or will mountains work? It can be hot anywhere in the summer, but the French Alps and the Italian Alps, Dolomites, and northern Apennines might offer some respite at higher, relatively cooler elevations.