"Are there three?"
The -ne is actually very important. In classical times, the Romans did not use any punctuation marks. Adding a question mark at the end is a modern thing (basically a guide to help learners of the language), and so adding a question mark alone is not enough to make it a question. Therefore, "sunt tres" would be a statement: "there are three". To make it a question, you need the -ne indicator. Alternatively, a different question indicator like nonne or num would be used (nonne tres sunt? num tres sunt?). But when no specific response is expected, -ne is used.
It should be fine. However (just in case anybody else besides me needs to remember what "-ne" means):
Particle -ne (interrogative enclitic)
Added to the end of a word in a phrase (usually the first word) to make it a question. -- Nihilne in mentem? (Does nothing suggest itself to your mind?) -- Ah, pergisne? (Ah, at it again?) -- Esne iratus, frater? (Are you angry, brother?) -- Potesne mihi succurrere, quaeso? (Can you help me, please?)
or… (introduces a question or an alternative)
- A question requiring an answer of "yes" or "no" is formed by adding -ne to the emphatic word: -- Is tibi mortemne vidētur aut dolōrem timēre? (Does it seem to you to be death that he fears or pain?) --Hīcine vir usquam nisi in patriā moriētur? (Shall this man die anywhere but in his native land?) -- Tūne id veritus es? (Did you fear that?)
*The enclitic -ne is sometimes omitted.
Patēre tua cōnsilia nōn sentīs? (Do you not see that your schemes are manifest?)
- When -ne is added to a negative word (such as nōnne) an affirmative answer is expected. -- Nōnne animadvertis? (Aren't you paying attention?)
[Taken from Wiktionary: https://en.wiktionary.org/wiki/-ne]
I'm curious why "tres" is used instead of "tria". At least when translating from the English "Are there three?" to Latin, I would expect it to be neuter (are there three things). Similarly, "is that all?" would be "Estne omnia". (Is that everything)
Going from the Latin to English, I think a better translation would be "Are there three people?". Because "tres" indicates 3 non-thingy things (i.e. people or animals).