The 3 theres.

Their There and They're

We all learned the difference between these three words in middle school, maybe you reviewed it in high school a bit, but once college comes people forget their purpose in the English language.
Before I get into the three there's, you should know a tiny bit about the parts of speech so you aren't totally lost. Here's a few of the parts of speech...
Noun: name of anything
Pronoun: takes place of a noun (he, she, it)
Adjective: tell the kind of noun/pronoun; describes noun
Verb: action word (I hope you knew that one already)
Adverb: tells how something is done; describes verb
Interjection: shows surprise
Contraction: combining two words with an apostrophe to make a shortened version

First off, their. It is a pronoun that shows possession. Often, it is followed by a noun. 
Example: Please bring the money to their house

Once you know the why of things in the English language, you can use the words right because you understand the explanation of their usage.

Next, there.  It's simple Use this one to refer to any place. It can serve as a noun, a pronoun, an adjective, an adverb, and even an interjection. See above for their definitions.
Example: Over there is where we can sleep.

Finally, they're. This one should be the easiest to remember its function. It is a contraction that joins they and are. So really it's just used if you want to combine these two words. It is the combination of two parts of speech, a pronoun (as the subject) and a verb.
Example: They're not ready for the exam. 
See how they're joins they and are? That sentence means the same thing as, They are not ready for the exam. So remember: if you can replace they're with they are and it makes sense, you got the right one.