Quick question, if I use Expose on my Decimator and then get attacked at Range 3 do I still lose the 1 Agility dice because of Expose's rule, or do I get the extra dice anyway?

Your agility can never be less than zero.

When defending against a primary weapon attack at range three your agility remains zero - but receive a defensive bonus of 1 green dice

The bonus green die for range 3 (and for being obstructed by an obstacle) is not a bonus to agility, so things that reduce your agility have no effect on it.