3 panels

Thank you!

A two finger pleated drapery or full gathered drapery is typically 2-1/2 times fullness. So with a window 78" wide x 2-1/2 times fullness = 195" divided by 48" wide fabric= 4.0625 panels.
Four panels would work plus length of finished height of window plus hems.

Some window treatments you can get by with less fullness than 2-1/2 times.

If you want 2 times fullness x 78" wide window= 156" wide divide by 48" wide fabric= 3.25 panels. You would still need 4 panel widths to make it work.

A typical commercial shower curtain is at 1-1/4 fullness.

So it really depends on what your window treatment is? If you post a photo of what you are thinking, I probably can figure it out by look at it.

4 panels - you would not get enough fullness with 3.

I agree with Ward Designs, 4 panels would be the best choice.

You're very welcome spur!