Most, thermal glider piltos will start to move the CG back a bit at a time over time. The further back, to a point, the more responsive the plane will be and the easier it is to read and work lift. Get it too far back and the plane becomes hard to handle.

If the starting CG is 2.5, then 3" would be a big move, but if you can handle it there and you are not having to work hard to keep the plane flying, then it it probably OK for you.