mysql select .. where .. in -> optimizing

I have a table with a number of columns and ~15K rows. Most important among these are three we'll call 'a', 'b' and 'c'. I have an index on these columns in that order (a, b, c).

When I do a 'desc select * from <table> where a = <some value> or a = <some other value>' Im told that it grabs the value with 2 rows. This is expected.

When I do a 'desc select * from <开发者_如何转开发table> where a in <some list of values>' Im told that it isn't using the index at all.

Why not? Is there some way to create an index such that 'where <column> in <list of values>' is indexed?