They’re is short for they are. They are going to the cinema tonight. / They’re going to the cinema tonight.
Their shows possession. It means it belongs to them. They wanted to give me their books.
There refers to a place. I’m going to sit over there now.
The trains to Ulm go every 20 minutes. The train to Timbuktu goes every three days. Every entspricht “alle” in Häufigkeitsangaben.