Question

What are the key differences between logos, ethos and pethos?

Answer 1

The Definitions of each

Answer 2

I can't tell the difference between when one is being used and another isn't.

Is there anything that will help me differentiate?

Answer 3

Ethos is the characteristic spirit of a culture,era,community,institution etc.,as manifested in its attitudes,aspirations,customs etc.;the character of an individual as represented by his or her values and beliefs;the prevalent tone of a literary work in this respect.
Pathos is a pathetic expression or utterance;a quality of speech,writing,events,persons etc.,which excites pity or sadness;the power of stirring tender or melancholy emotion;physical or mental suffering
logos is The Word of God,the second person of the Trinity;A pervading cosmic idea or spirit of creativity or rationality

Answer 4

This is a pretty reasonable thing on each and not too long:
http://www.calstatela.edu/faculty/jgarret/3waypers.htm