To be clear, those 3 words have different meanings.
“their”
This word has a meaning. It’s used to say that someone’s object has be mentioned/identified by the person speaking. For example: “Looks like someone dropped their phone.”
“they’re”
A contraction used by the words “they” and “are”. The two words are also used in some sentences.
With “they’re”: “They’re taking the kid to the park.”
With “they are”: “They are taaking the kid to the park.
“there”
A common word used to know where the place a person has been at, or has been at, or is in it. Take this for example: “If you’re looking for assistance, it’s over there, at the front.”
I hope this helped you. (^-^)v