Finding the closest value

[Lee Goddard]

Is there an efficient way to find the closest numerical value, across 
three columns? I do not know if there is a way to find it across one.

I thought about plotting the values in a geometric space, but that would 
seem to require more dimensions that the server can handle. I thought 
about using a Kohonen network, but I would prefer to use MySQL.

Many thanks in anticipation
Lee