It was six weeks, not three days.
https://en.wikipedia.org/wiki/Battle_of_France#Battle
To put it simplistically, the French thought that the Maginot line was impregnable, and the Ardennes were impassable. The Germans passed through the Ardennes, cut off the Maginot line, and completely crippled the French ability to wage conventional war. After occupation, the near-insane heroism of the French resistance played a large part in the eventual German defeat.
The Dutch were not expecting to be invaded, and were completely outclassed in the air. The Germans bombed Rotterdam to the ground and threatened to do the same to the rest of the Netherlands unless they capitulated.
Spain and Portugal were themselves fascist dictatorships. Perhaps the only reason Spain didn't join the Axis was the fact that it was still largely devastated from the Spanish Civil War, which only finished 5 months before Germany invaded Poland.
Yugoslavia collapsed after the German invasion, but the Partisans, with logistical and air support from the USSR and Western Allies kept fighting, and by the end of the war had largely driven the Axis forces out.
Greece resisted till the last, successfully repelling an Italian invasion before being overrun by the Germans.
There is no evidence of Ireland refuelling U-boats, and in fact we allowed Allied aircraft to overfly Ireland, using the Donegal corridor, in order to search for U-boats.
The whole "Britain stood alone, so you should be grateful" shtick, well, while their resistance was no doubt admirable, if it wasn't for an accident of geography in the form of the Channel, they would probably have been steamrollered by the Germans in just as short a time.
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