"Three, five and six are my numbers."
Translation:שלוש, חמש ושש הם המספרים שלי.
That's very observant. Maybe it's because מספרים is masculine. Here's a paper on the topic: https://faculty.biu.ac.il/~grinbey/pub/predication_and_identity_in_Hebrew_copular_sentences.pdf
Why הם and not זה? When there are pairs of numbers in a nominal sentence (e.g., 9 is less than or more than another number) I've noticed that the copula is זה but here it's הם. Is it הם here because the second part of the copula is not a number? Rivka Halevy has a discussion of "Deixis: Modern Hebrew" in Encyclopedia of Hebrew Language and Linguistics (Bill, 2003) pp. 693-97, here p. 697, in which she writes that in spoken Hebrew, זה is frequently employed as a stylistic variant of the third person pronominal copulas הן, הם, היא, הוא. Since זה can sometimes be used for הם, why not so here? Would it be problematic to use זה here or just very informal?
I intuitively wrote "הם" because they are more multiple numbers, not just one. "9 (it) is my number." vs. "3, 5 and 6 (they) are my numbers." It doesn't make that big of a difference that they're numbers. One could also say "NameA, NameB and NameC (they) are my siblings/children/dogs/teachers".