this sounds very much like a homework assignment....
aside from that...
you want to do a few things....
you said only three specific values.... as you suspected, read about CHECK CONTRAINTS in you books online.
you might also want to make the column NOT NULL when you build the table, so it cannot be blasnk either.
to force a column to be unique, you'll want to read about UNIQUE CONSTRAINTS in books online as well.
the combination of those three should get you where you want to go.
show us what you've built so far and we could offer some more insight.
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