Are Ruger #1 and #3 barrels interchangeable?

According to members of the Ruger forums, yes. But the headspace might need to be checked / adjusted:

https://www.rugerforum.com/threads/ruger-no-1-and-no-3-parts-interchangeable.296282/

This is my #13 custom that I built. They say a picture is worth 1,000 words.
It is a 22 Hornet #3 converted to a 7mm RM with all #1 furniture and barrel. The #3 lever has been cut to use the #1 butt stock. The extractors are the same for a #1 and a #3. So if you can convert a #3 to use all #1 parts, except the lever, you can do the reverse if you should want to do so. The rifle shoots sub 1/2" groups using the 145 accuracy load straight out of the Lyman manual.

Personally, I would rather re-barrel a #1 with a new or take off #1 barrel, that is headspaced properly.

Got a B in .22 K hornet comin.
For rebarrel to ?

The #3 and #1 are both have barrels setup on the centerline of the axis of the action. With a change of the barrel and the extractor they can be pretty much be anything you want, as long as the case diameter fits inside the minor diameter of the barrel's action threads, plus a tad of safety thickness. Butt stocks are the same, forearms are different for the barrel diameter and decoration, but generally the same, a fairly minor duplication cut item if needed.
If you have slave pins the action can be removed as one piece, trigger job done and then replaced as one piece.

Thanks for the replies guys. Another gun head I know picked up a Ruger #3 that has a Ruger 243 WIN barrel installed on it. The stock has been shortened for a youth. Hookeye, I saw your reply on another thread about conversion to 35 REM, that sounds tempting to me.....

Yeah, im wondering if my hornet B could be rebored to 35 rem, cut to 22" and just add an Alex Henry forend.

It may get a new stainless .243 bbl in factory contour/length

Anyone have a Ruger #3 action for sale? 7X57steve was a member here. He passed away several years ago. He had a custom #3 in 25-35, I bought it from his estate. The family found the original 375 Win barrel that came on the rifle and gave it to me. Now that the 375 is legal in all Counties in MD, I'd like to have another #3, in 375.