Okay, so, I think I have a pretty good handle on how these properties are separated. However, if I'm wrong, please tell me.
Memetic- Gets in your head and won't leave. Doesn't really make you do anything, anymore than a very persuasive person who is persistent would.
Cognitohazard- Actively messes with your thoughts. If you think something you wouldn't normally because of an SCP, or if you don't think things you would normally, that's a cognitohazard.
Infohazard- Spreads around things people should not know. All this does is give you information, what you do with it is up to you.