Answer:
sentence #2
Step-by-step explanation:
A VERY SIMPLE explanation.....
A simple sentence is an independent clause that expresses a complete thought.
Sentence #1 is a complex sentence; it is made up of a dependent clause and an independent clause.
Sentence #3 is a compound sentence; it has two simple sentences (independent clauses) connected together.
I could elaborate with much more information, but that might be too much information for this simple - no pun intended - question.