1. They’re=They are. If you can’t substitute “they are,” you want a different form of this homophone.
They’re arriving an hour early to make sure to get good seats at the concert.
2. Their=Possession. It indicates ownership. The word “heir” is contained within this word, so you can remember that an heir will inherit possessions.
The campers retrieved their food bags from the pine tree and cooked their dinner, hoping not to be threatened by bears.
3. There=Location. It is similar in spelling to “where” and “here,” words that also indicate location.
I love visiting New York, but I haven’t lived there for a very long time.
Be sure to proofread your writing to make sure you have the correct form.