First off, remember that the size of the wheel is the diameter. ie going from a 2.5" to a 3" maybe 1/2" up in diameter but it will only give you 1/4" extra clearance.

Another option which may give you more height is to simply bend the landing gear. The main gear on the Superstar is probably quite wide and could be bent in a little for more height. bend both ends to keep the wheels perpendicular. Then you should also be able to adjust the height of the nose leg also.

I don't recommend putting the plane in a nose high attitude. When I put my LT-40 together I had to use a nose leg from elsewhere as the original wouldn't fit. The replacement is a bit longer resulting in a slight nose high attitude. I'm finding that when landing, I have to keep the nose up to for a proper 3 pointer. This makes it tricky to keep the plane down when you're still carrying some airspeed.

BTW I've also got about 2.5" clearance. The length of the grass isn't the problem, its that the runway is a bit bumpy it could either be the unevenness of the ground or the spring in the nose gear causing the prop to hit.