I never saw any discount items before, (there was also a thread about giving discounts to attract customers, which was pretty neutral i guess)...
Maybe they are giving the $3 from their end? idk, but i'd like to know more
No Envato are not reducing their commission with this promotion - they will keep their standard percentage and so will you, meaning that because Envato want to have a sale with your work you and they will both earn less.
Not your choice unfortunately, as already stated in other threads they have full right to alter product prices at anytime - they could sell it for $3 (or $30) and there is nothing you can do.