"We have three children."
Translation:У нас є три дитини.
Because it’s not used this way. With cardinal numerals ending in два, три, чотири we use the form дитини.
The same is true for other nouns with the suffix -ин- that lose it in the plural: три люди́ни 'three people' (plural is люди), чотири львівʼянина 'four peope from Lviv' (plural is львівʼяни), etc.
(There is another group of numerals, collective numerals. Those use genitive plural: двоє дітей ‘two children’, троє людей 'three people', четверо львівʼян 'four people from Lviv'. But you don’t need to worry about these numerals, at least if you’re just beginning studying Ukrainian. Collective numerals can be replaced by cardinal numerals in almost any context, but the reverse is not true. So, it makes more sense to learn cardinal numerals, which is what this course teaches.)