Duo should accept "euro" as the plural, according to [https://en.wikipedia.org/wiki/Euro#Linguistic_issues]
On the other hand, this would seem to indicate that "eŭroj" is correct: https://en.wikipedia.org/wiki/Linguistic_issues_concerning_the_euro#Esperanto
Correct too, looks like the EO plural is always "eŭroj". The English plural should be "euro" according to the institution that actually issues the things, with "euros" having some support as an alternative, and most common use depending on the country. I'd go with the Irish "euro" plural as carrying the most weight, Ireland being the only English-speaking country in the eurozone.
Fair point, but in those cases 'euro' is a modifier for 'coin' or 'note'. It's not the main noun and thus the situation is markedly different. Which doesn't mean that I'd dispute your claim of 'euros' sounding odd. Just that I too would say 'a two euro coin'. It's only when 'euro' is the main noun (say, in 'a coin worth two euros') that I'd use the s-plural.
Duo is trying to correct "They cost €3 each" to use 3€ instead for the English translation. Sure, putting the sign after the amount is standard practice in Continental languages (and so is probably the standard for Esperanto), but in English currency signs are always put before the amount for all currencies, including the Euro.