Are we a two or three part being?

There are many people who teach that we are a two part being, having a body and a soul.  Others teach that we are a three part being, having a body, soul, and spirit.  So which is correct according to the Bible?  Those who say that we are a two part being state that the words soul and spirit are used interchangeably in scripture.  But this is not the case.  We see that the soul and spirit are used in the same verse many times and considered different (1 Thessalonians 5:23; Hebrews 4:12; 1 Corinthians 15:45).  Through these scriptures we see that we are a three part being; created in the image of God who is as well through the trinity.