Need help making concentric arcs in Illustrator

[candlefish]

I'm attempting to make three concentric arcs, each one on top of the other at equal distances from each other. I started off making an arc with the pen tool, then scaled it larger and copied it. The problem is that each time I scale and copy the arc, the new, larger arc changes shape slightly (not just size) and doesn't match the smaller arc below it (the curve is a bit different). Any help with this would be really appreciated!