Waggy,
I have a set trigger on one of my Sako 85's. Mine was fitted at the factory, but I did some research about having it done retrospectively and found out the following.
My understanding is that the whole trigger group is replaced. The task involves unscrewing the old one and replacing it with the whole new unit. I am told it is an easy job for a rifle smith. I assume it is the same procedure for a T3, but you would need to check. Chris Blackburn from UK Gunworks does alot of good work with Tikka's so he would be a good place to start.
Apologies for this next paragraph if you are an experienced shot and know what you want, but what is your reason for wanting a set trigger ? The reason I ask is that I got one because they were available and I had to wait for the rifle anyway so I thought "what the heck, I'll have it". With hindsight, I wouldn't have bothered.
The set pull on my Sako isn't (as far as I'm aware) adjustable and it is VERY light. If I'm careful, and under good conditions (warm hands etc), I can rest my finger on the trigger blade without setting it off. However, the slightest pressure beyond that and its away. I very rarely use it. I would certainly never use it in the field. Many experienced guys advise that a well set up normal trigger which breaks crisply at a safe amount of pull is the way forward. Based on my own experience as a relative novice, I am minded to agree. The trigger on the T3 is pretty good as it stands. You might be better off getting a smith to adjust it down a bit instead of the set option.
I apologise if you've had loads of rifles and the above is teaching your nan to suck eggs !!!.
Cheers,
Bob