I also just added a Ruger Mark III to my collection.
I shoot rimfire in Steel Challenge and decided to go the right way instead of using my AA on top of a Frank-en-Glock.
I purchased a trigger upgrade kit from Volquartsen (actually through Brownells).
I than spent 3 days with tow other shooters bring to get the kit in the lower.
The trigger goes in nice and easy, but if you are upgrading a Mark III, and want to do away with the Mag Safety, you have to change the sear and Hammer to a Mark II parts. Because of legal ramifications, they can not tell you to change those two parts.
After a call to the manufacture, I sent them the lower, they put in the kit, replaced the sear and hammer, set the trigger to 2# with no under or over travel (at least none to be detected).
With in two days it was back in my hands and ready to go.
My Mark III Custom is outfitted with a C-More, Allchin low profile mount, custom grips and ready to go.
Sighted in at 50 & 75 feet it will shoot 1.25" groups.
And here it is:
