How to validate select field in post meta?

Separate the allowed values in a function:

function get_allowed_values()
{
    return array('one', 'two', 'three');
}

In your metabox:

foreach ( get_allowed_values() as $item) {}

In your validation handler:

if ( isset( $_POST['select_meta'] ) 
    and in_array( $_POST['select_meta'], get_allowed_values() )
)