Excellent find!!! Thank you for posting it.
3-ply is somewhere between 4-ply and lace weight.
3 ply yarn is finer than sports weight, which is about equivalent to 4 ply. I knitted with it for my children when they were babies. I still have some somewhere in the stash. It's one or two up from lace weight! It does not matter too much about gauge for shawls. Here's a free Beehive booklet from from the Internet Archive. Many of these manufacturers donated their old patterns to the University of Southampton to digitize and make available to the public; there are others on there but I just found this one on a quick search: https://archive.org/details/Patons_SC50_Beehive_baby_bookhttps://archive.org/details/Patons_2105_Heirloom_Collection_Baby_Rugs_and_Shawls
This hoarder has saved patterns dating to the 50's... (
Thank you Celt Knitter for your research. It's good to know that these old patterns are being preserved.
Celt Knitter wrote:
3 ply yarn is finer than sports weight, which is a... (
Thank you for those links!!