Weight and balance question.

My plane, a 175B has an O-360 engine and a constant speed prop. I had
the plane weighed and and about to begin the calculations of finding the CG,
and it's measurement behind the datum. I'm a bit rusty there and would
appreciate what ever help you could do. Here is what I have.

While plane is leveled according to specs
Left main gear 644 pounds
Right main gear 631 pounds
Nose gear 528 pounds
Total, 1803 pounds.

I'm not real sure how they get the CG while the left and right weights are
different. Do they average them? I don't have the measurements for the gear
with me at the moment, I intend to use the standard values for the plane
rather than measing my particular plane.

What further information do I need? The shop is going to calculate it as
well, but I want to understand it as I do it. Thanks in advance for your
help.
Wayne