I was wondering if there is an easy way to access the 3D editor without starting FSX?
You basically asked if it were possible to use FSX without FSX...what makes the 3D editor "3D", it's the fact that it runs under the FSX engine.
Of course, you can always edit an airplane configuration without using FSX, by editing the GSX.CFG file directly, which is what had to be done before the 3D editor was added, but of course it won't be so easy.
I really don't see the point of editing an airplane outside FSX. You would have to *exit* from FSX, edit the airplane, and enter it again, which is far more cumbersome than just editing the airplane you are flying now, without exiting and re-entering.
Perhaps you missed the fact that, when you edit an airplane configuration, you can have it applied immediately, by using the "Restart Couatl" option ?