RE: 3" feathers with mechanicals?
I've been using the 4" and 3" vanes, three on an arrow, with the Rocket Steelheads for a while now.
I've tested them out to 50 yards and can get them grouped pretty well, enough to stay in the vitals at that distance.
It takes some time to tune your bow, it's a hobby I enjoy and spend allot of time at. This is one of the better ways for us to achieve good results.
The more you get to know your bow the higher confidence level will be.
Practise, test--change this & that and you will eventually get the right set up and what you're looking for.