The 3 times is over the entire measurement of the window plus however much you want to cover the walls, or the entire curtain. In other words each of the 2 panels would be 42x3.

This is just a guideline. The rule for fullness in curtains is 2 to 3 times the width you want to cover no matter how many panels you have. The heavier the material the less fullness is needed. A light or sheer material will require 3 times. A heavy material only 2 times. It just depends on the fullness you want.

Will you be closing the curtains?

For an 84" window, I would use 1 and 1/2 widths of 54" decorator fabric per panel.

If you need help with fabric quantites,let us know.

This treatment does need 3x fullness but you should be measuring the rod, not the window. If you're going to open them you want to have some stackback, so the rod should be wider than the window.
Any rate, you mulitiply the rod width, then divide by 2.

Yes I will want them closable. Thank y'all so much for helping me.