Translation:Three glasses, please.
Still can't get with this "can I get" thing for ください. Still much better than "want", I suppose, as it's translated in a few other example sentences, but a simple "please" would still work better for the same meaning, without confusing people. Especially when it's used in so many various conjunctions where it has nothing whatsoever to do with "getting" anything.
Maybe because in English, just saying "cup please" or "three cups please" is a bit rude. So since "カップを下さい" is a polite sentence, they translated it to a polite english sentence?
I'm just guessing.
They should have a literal translation as well as a translation that is gramatically correct in english.
The word "kudasai" literally means "please give me". "Can I get" is an incorrect translation, it leaves out the word "please" and implies you'll get the item you're asking for yourself. When I told a bilingual friend (Japanese/English) about the translation "Can I get", he laughed and said "no, that's not quite right". The "Can I get" mistranslation is prevalent throughout the Japanese course, how can we get this fixed?
"KUDASAI" means "PLEASE GIVE ME". FIX THIS, KUDASAI.
You are right about the reading for 2 cups being futatsu. One different thing about Japanese is that there are many "counters" that are used for different types of objects. You use different counters for pencils, animals, cars, books etc. I haven't seen something in Duolingo that addresses this for new language learners.
Considering there's the honorific "o" already in use for romanization, I use "wo" to indicate the "wo" particle. Also, I usually have a tiny bit of "w" when I pronounce the particle. Not really noticeable, though, when you heart it. However, I never romanize the "ha" particle as "ha." I always say "wa."
the particle を is marking the object you are selecting or affecting with your decision. So コップを is whole in this example. You can move it for example as 「三つのコップをください」but you need to be aware of the grammatical structure.
Also as percuriosus stated in this thread the expression [quantity+verb] is an expression of extension, meaning the quantity is acting as an adverb modifying directly the verb or the whole sentence. Similar to how you can say 「コップがたくさんあります」"there is a lot of glasses".
So, I know you didn't ask for this but I will still explain it here for anyone wondering about the difference between:
「(noun)(particle)+(quantity acting as a direct modifier)→verb」
So let's use an example from real life, for example this show called "Nanatsu no Taizai" or「七つの大罪」the expression is similar to how we use "the" to delimit the number of things you are counting, is not any seven deadly sins, we are talking about these particular seven ones; As in english the difference can be seen as comparing "The Seven Deadly Sins" and "Seven Deadly Sins". In english "The" acts as a delimitation of a group.
So the difference between both structures in this example could be translated in english as:
「コップを三つください」"can you give me 3 glasses please?"
「三つのコップをください」"can you give me these 3 glasses?"
So with the second structure you can express delimitation but in general is not always the case, both structures mean basically the same in japanese.
It really depends of what you are ordering. When ordering one type if stuff japanese people usually use different counters and they use these to know what they are talking about, sometimes you don't need to be specific about the container like in english.
for example「ホットコーヒーを3つください」would mean "3 cups of hot coffee, please", even though there is no カップ in there.
「水を2杯ください」杯【はい】 is the counter for liquid in glasses, cups, etc. So this would mean "Two glasses of water, please".
But yeah you could theoretically say「コップのミルクを3杯ください」and that means "3 glasses of milk, please". Although they would usually just say「ミルクを3杯ください」and it means exactly the same.