I feel this is still a tuning issue. However, 6" low and 8" right is a lot of adjustment. But, that 3 blade is also 18.75% wider than the 1" 4 blade, that's a bit more planing area. You would have to move your rest to the left and your nock set down to get them together. With that much adjustment you'll have to resight your bow also, however you'll be hitting all 3 tips together. Sounds like your form is ok since you're getting consistant groups also. This late in the game I would be inclined to stick with the 4 blade muzzy's. However, it would only take an afternoon to get things set. As for a spinner? Well, I made my own out of a long board, 2 joist hangers and 2 sets of drawer rollers. I also put a line on a pick of label at the front that is .3mm wide and tune my broadheads to center that line when spun. It works great but takes a little time to make. I find it to be better than the commercial ones (doesn't look like it though). Here's a picture. It is well worth it to get the wobble out. Your broadhead groups will be as small as your field tip groups. Just remeber if you move your rest and nock point to set one at a time so you can always go back if you make a mistake.

Arrow Spinner