Just how many lights are there under each of the fluxing tubes? I just don't know. There are at least 4, but there may be as many as 6 from what I've read on other BTTF web sites. While I'm sure having 3 would probably be fine, if I'm making the circuit from scratch, adding more lights is not a big deal.
I decided a little investigative work was in order. I popped in my BTTF DVD and imported the Flux Capacitor clip into my video editing software. It didn't take to long to find the right scene. There is a key moment when Doc Brown points at the Flux Capacitor for a few seconds and explains to Marty, holding the camera how time travel works..
I captured the clip and looked at several consecutive frames showing the fluxing lights. Now, I know all about film speeds and frame rates from 20+ years working for Kodak and there's just so much you can gather from looking at a DVD, frame by frame.
I figured I'd count the lights I can see in each frame and plot their positions.
Click on the picture above for a closer look. I stepped through every frame I could and marked each light location that I saw. Except for occasionally seeing two lights at once (one turning on and one turning off) I never saw more than the 4 positions I have circled. Even considering frame rates and the possibility of missing a light, I'm not sure there are more than 4.
If you look at the spacing between the 3rd & 4th light (counting down from the top right) there may be enough room for another one. Also the first three overlap a little but #4 is all by itself.
So, the jury may still be out for some, but I've settled on 4 lights.