English tends to treat numbers as adjectives, so we say things like "There are 3 dogs" or "I have $3,000" as opposed to "I'd like 4 of hot dogs, please" or "Give me 3 million of dollars or the cat gets it!"
Numerous counterexamples exist where numbers can be nouns, but they are mainly expressed as adjectives.
You are absolutely correct... about English. But Spanish (unfortunately, from a learner's point of view) is not English. So in Spanish numerals up to a thousand 'act' as their counterparts in English, but from million upwards they 'feel' that they should be treated as nouns.
De can translate as "about", but not here. How would you even incorporate that in this sentence?
You should avoid trying to directly translate prepositions. They always have to be viewed in context. Here it's used because millón and dólar are both nouns, but in English they aren't, so the de remains untranslated.
English has the fun ability to mash two nouns together and combine them into a single "complex noun". "Window" and "glass" become "window glass" and so on.
You usually get told that "the first word is an adjective" in these combinations, but you don't usually get told what that means. Here's what it means: English adjectives don't have plural forms. You have a "student council" even though it's intended for multiple students, and at a "book sale" they sell multiple books.
This lack of a plural form becomes blatantly obvious when you add a specific number. If a steak costs nine dollars, it'll be a "nine-dollar steak", and your "eight-hour job" makes you work for eight hours.
Who are you talking to? What are you talking about?
In case you are not familiar with these forums: you are speaking to your fellow learners. We do not have any information; if you want any help here you need to provide it.
I assume that you have a problem with a task. Given your XP level and streak number I assume you are not a novice and you know how to read tips and do exercises.
Was it that you do not use 'de' while translating in Spanish: "Tenemos tres millones de dólares"?