If it were mine, I would add a 4th. But I would make it a much smaller, less conspicuous wedge with no escutcheons. Like an 1/8” thick x 1/4” wide, or there abouts. I would even skip the head all together and just have a small rectangular wedge that was tight in the loop, and flush with the sides of the wood fore end. You could hammer a very slight rolled edge to hold it in place.
You could also use a pin. But a pin would sort of defeat the purpose of an easier barrel removal with the the wedges. My 2 cents.