The differences between the three are very slight.

Psychic - pertaining to mental forces, telepathy, extra sensory perception.
Medium - contacting and being able to communicate with spirits of the dead
Clairvoyance - the ability to see things beyond our normal senses

Psychics are more general in what they can do.  They claim they can predict the future or "see" things that others cannot see, kind of like someone who reads palms, crystal balls, tarot cards and tea leaves. 

Mediums talk to dead people.

Clairvoyants find missing things.

All of them are condemned according to the Bible.

“Do not turn to mediums or seek out spiritists, for you will be defiled by them. I am the LORD your God." Leviticus 19:31

A Spiritist is one who consults the spirits or allows himself/herself to be possessed by a spirit.  Similar to a medium.

He sacrificed his own son in the fire, practiced sorcery and divination, and consulted mediums and spiritists. He did much evil in the eyes of the LORD, provoking him to anger. 2 Kings 21:6

All of those practices of sorcery, seeking out the divine in things, consulting other spirits are seen as detestable and make God angry.

When men tell you to consult mediums and spiritists, who whisper and mutter, should not a people inquire of their God? Why consult the dead on behalf of the living?  Isaiah 8:19

Anyone who consults spirits instead of God (who is Spirit and created all the spirits) is putting another god before Him.