Chasing a yew sapwood ring is actually relatively easy. Because your stave has pretty wide rings, AND very few knots, it'll be a perfect candidate to chase a ring on. Even doable for a beginner.
I suggest you narrow the stave somewhat first. It makes no sense to chase a ring over the full width of 3", while you may end up with only 1/2" wide tip. It would be best to rough out the bow to the correct width, but to leave it 1/2" wider. Then thin the sapwood, and only then come down to the final width profile.
When chasing a ring in yew sapwood, I would first count rings and check the thickness over the entire length of the stave. You can either have a continuous sapwood thickness, and the cost of cutting through some rings. Or you can follow one ring faithfully, resulting in slightly uneven sapwood thickness. Myself, I usually go for the latter. If that means the sapwood will be very uneven, you could "step through" a few rings in the stiff handle. That may mean that one limb consists of six rings of sapwood, and the other limb has nine rings.
