Дити́на is singular, ді́ти is plural.
But after numbers ending in 2, 3, 4 (but not 12, 13, 14), you use дити́ни. This is actually illogical. In the past, Ukrainian had a dual number. When it disappeared, most dual words were replaced with plurals, but after numbers they were replaced with the forms that sounded similar. For most nouns, this was the nominative plural form, but дити́на is an exception.
(To further complicate the matter, dual forms became used with 3 and 4, not just 2. Because people started to think these are forms for small numbers.)
Gee, I would have put: "Ми маємо три дитини." OR "Ми маємо троє дітей." [If there's both boy(s) & girl(s).]
"У нас є три дитини." sounds very awkward. It sounds like you're talking about 'someone else's children' that are staying at your home (impersonal), rather than 'the children of your own family' (personal).
Just an observation....