How do I calculate the CG of my airplane, using three scales?

Here's how to do it:
You must block the plane up level.  Assuming we're talking about a tail dragger like the Edge, you will have to block up the scale under the tail wheel.
Carefully measure the distance, parallel to the plane's longitudinal center line from the spot the mains contact the scale to the spot where the tail wheel contacts the scale.  Call this distance (D1).  Take the total of the weight on the mains and call this (W1).  Take the weight on the tail wheel and call this (W2).
In the following equation, the distance to the CG (Dcg) is the distance from a line, perpendicular to the longitudinal center line and passing thru the center of the mains contact spot, aft to the CG spot.

Dcg = D1 x W2 / (W1 + W2)

Example:
D1 = 90",     W1 = 30 lbs,     W2 = 5 lbs

Dcg = 90inches x 5lbs / 35lbs
Dcg = 12.86" back from a line thru the mains along the fuse center.

The principle of the formula is that at the CG the sum of the fore and aft moments will be in equilibrium.  We know the forces (Weights) and the distance between them (D1) and have to solve for the spot where the moments are in balance (Moment = force x distance).
You can also use this formula to tell how much weight you need to move to get the CG to move a given distance.  I also have a little Excel sheet that does this. Mike Stoner