The Mark Two kit works in a Mark Three when the magazine disconnect has been replaced with the Mark Two hammer bushing. That's exactly what I have in my Mark Three.
Here's the Mark II hammer bushing:
http://www.brownells.com/.aspx/pid=31838/Product/RUGER_reg__MARK_II_HAMMER_BUSHING
And the page with the Mark II accurizing kits, I did the kit with the stainless trigger, fairly new item:
http://www.brownells.com/.aspx/pid=13864/avs%7CManufacturer_1=VOLQUARTSEN/Product/RUGER_reg__MK_II_DROP_IN_ACCURIZER_KIT
You get the most benefit from the trigger, sear, and hammer. But, by the time you total up their separate prices, the whole kit is a very few dollars more.
They advertise the kit giving you a 2.25 lb trigger. I used the Ruger springs instead of the VQs in the kit, and that left my pull about 3 lb.. Just depends on what you want. I didn't use the extended bolt thumbpiece, either. Looked like it may be a bit rough on holsters.