3" is not an issue, rocker wise. The length of Tom's boats vary from builder to builder.

One can simplify the building process by using the strongback as your baseline, btw. In my pic, the two center frames are at 0" HAB - so I only need to make sure the keel sits at the correct HAB at the first and last frame, then I can just strap down the center frames to the strongback. No fiddling necessary, and I know it's not going to move around when I'm building.

Good luck!