I used to own one in .223 and thought about changing calibers.

I think all you need to do is change the barrel and maybe the extractor. If you are changeing calibers with the same case head you wouldn't need to change the extactor, ie. .243 to .308 to .270 to .30-06 etc.

I am pretty sure this is all you need to do. But I never got past the "thinking-about-it" stage.

I know they made them in 45/70. And I think 22 Hornet. (though I may be wrong on that last one)

Remember, and I'm sure you do, that you're not limited to whatever the #3 was chambered in. You can use any barrel from a #1 also. They are the same action. They just have different levers and stocks.

My father and uncles bought my grandfather one in .22 Hornet back in the late 60's. My father got it upon my grandfather's death in 1992. I had it in my safe for a few years before returning it to my father. I cannot remember what year they were discontinued, but I believe it was around 1982. I called Ruger and got an owner's manual and parts list for it. All paperwork sent from Ruger was stamped "Out Of Production" on them. I also found a set of scope rings and bases for it 3-months and $85 later. My father took them off, as he prefers open sights. Sorry I could be of no more help.