Also known as a Stewart Tackle used for salmon fishing. Unfortunately often used for foul hooking fish but originally like the Pennel tackle (2 hook version) they were intended for use with worms.
Yes worming for salmon and sea trout. I'm not sure about the former but if fresh run the latter can have soft mouths and they do tend to pluck at the bait. I've only ever used a single hook though when worming.
" I find what is called 'Stewart's tackle' is the best. This consist of 3 small hooks about an inch apart, two facing one way and the centre hook the reverse. Some fish with 'Pennell's tackle which is simply 2 hooks further apart and some even with a single and consequently larger hook, -------"
Extract from - Clear Water Worm Fishing by Philip M Boase in 'FISHERMAN'S PIE' dated 1929.