The answers are no, no and no.

The only way to change the way a model looks in G3 is to create one from scratch using the new update tools (that aren't released yet) and 3DSMax (which costs $3500).

Microsfoft "Paint" will not allow you to edit "targa" files which is what G3 uses for texture mapping the planes.

You will need to purchase a Paint program. I use (and reccomend) PaintShopPro cause its cheap ($90-110 price range) and works great. Adobe Photoshop is also an option but is considerably more expensive ($649.00) and is not any better than the current version of PaintShopPro. Like anything else their is a learning curve to painting aircraft and it depends alot on how fast you pick up on things.