On a tandem bait there's a blade at the end of the upper arm hanging over the hook. Sliding on the upper arm is usually a half circle metal clevis holding a normally smaller blade. I remove that inside blade and add a treble hook. If the blade is cheesy cheap I'll cut it off the clevis, then add a split ring to the clevis, then the treble hook (not using a swivel). Otherwise I might cut the clevis off to save a quality blade so I use other blades, attaching hook directly to the arm with a split ring. If not using the hook, I just clip on a blade using the quick plastic clevis, already made up at home.
When custom building I use a metal clevis and split ring with the snap swivel for blades, and can unsnap a blade, then snap a hook on.
Another reason I don't go with the regular spinnerbait barrel swivel and split rings is I lose too much time fooling with split rings in the boat, especially in low light. If you have good eyesight and are handy with split ring pliers, you get better spin or vibration sticking to the stock swivel, depending on which way it aims, toward the blade for smoother spin at lower speeds, away for more vibration at higher speeds. But it only takes me a couple of seconds to switch a blade and I have more confidence in the better hardware. I spend a lot more time finding the blade I want. I think I sold my last custom spinnerbait, but will keep looking and try posting a photo. The hard part is doctoring it up in a photo editor.
The main advantage of putting a treble hook in the place of that front blade is to get better hookups without the misery of adding a trailer hook to the main hook. Those get snagged too easily and a bass gains extra leverage from lengthening the lure. The bass can throw that rig easier. But not if any of that treble grabs mouth along with the main hook. The treble is protected by the upper arm. Of course, since at least two points of the treble stick out the sides, it's more prone to hangups in wood unless you use weedless trebles, the short ones with the three wire guards.