French language is always tricky, so I’ll say, both and more 🙂

If “Eux” is used as a subject, it’s they. Ex: “Je voulais venir, mais eux ne voulaient pas” => “I wanted to come, but they didn’t”.

If it is used to induce a possession, it’s their. Ex: “Je vais chez eux” => “I’m going to their house”

Finally, you can also find it used as an object, and then it’s them. Ex : “Tu les as vus?” => “Have you seen them?”