Because you want to be able to purchase 1 - 3 sizes at the same time but show the $30.00 as the starting price or a Special Price of $25.50, you need to configure this with one of the Attribute Prices to be used for the Price by Attribute ...

For example, if you set the Attribute on the 6mm to have the:
Include in Base Price
When Priced by Attributes YES
and the other sizes to NO then you will see the Display Price as $30.00 ...

If you put the Product on Special for 15% off, you will see the Display Price as:
Starting at: $30.00 $25.50
Save: 15% off
with the $30.00 crossed out and the Attributes should each show as: $25.50 ...