Sprocket spacing won't be critical if using a non-indexed gear shifter (lever). Otherwise they will need to be spaced appropriately for the shifter and derailleur.
You need to work out the total width of a normal cassette on your hub = n X sprocket-thickness + ((n - 1) X spacer-thickness), where n = the number of sprockets the freehub normally takes. Call this t.
Then calculate the width of your 3-speed set. Do the above with 3 for n. Call this s.
You'll probably want to have the sprockets roughly in the middle of the freehub, so the total thickness of spacers needed either side = (t - s) / 2. It will only need to be precise to within about 1mm, but add one or more thin 0.5mm spacers if it's a tad too thin for the lockring to lock.
You can use any Shimano or SRAM sprockets butchered from any cassette compatible with that hub, and any make of spacers of whatever thcinesses you need. Metal spacers, if you can find any, will help to reduce flex. The amount of flex that a load of plastic spacers together will allow might be more than you want.
Use any "speed" of sprockets and chain you like. Just make sure there's enough room for the chain not to rub neighbouring sprockets.
Good luck with your bonkers idea