I looked at your project when I was doing my research...and watched your video...awesome!
Lemmy was the MAN...it was a sad day at our house when he passed.
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I think you're on to something.....
Hmmm...I wonder if there's room for four mm pups...that would mean more knobs :D
...a headstock logo.
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Yes? No? Too cheesy?
A 6 position rotary switch (the largest available that I've seen) will give you all but one of the seven possible parallel combinations of the three pickups. Which combo you leave out would be your choice, typically either the 'all 3 on' or the 'bridge + neck' combo if you were wiring a Strat style build.
If you use two rotary switches, particulalry the 4-pole type, your possible combinations and options expand significantly. Then you have to decide what combo's you actually want (or are likely to use) and we can start working out how the switches can be wired to achieve that. As Simon mentioned earlier pickup combinations plus parallel, series, phase options etc.
Go digital switching, the possibilities are endless.....
Another option, maybe a bit simpler is to do what I did with three pickups on my Bass IV kit - a individual stack knob with tone and volume for each pickup, and then on/off sliders for each one.
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Wow some big plans here - this will be one impressive beast - i like the colour combo you have chosen
That's five single coils, so five sets of magnets. 3 humbuckers = 6 sets of magnets. That's 1 more. Each magnet does have a braking effect on the strings, that's just physics. Maybe it has less overall effect on bass strings than guitar strings, with them having more mass and momentum. Setting the pickups a bit further away from the strings helps to increase sustain, and pickups with weaker magnets will have less effect than more powerful magnets. But add in a compressor and the apparent sustain goes up. Add in more volume and the sustain level goes up. So there's a whole combination of things that can affect the sustain.