The 3 speed cassette i made was using 2 single speed cogs and 1 from a normal 8 speed because i could not get a 28t single speed cog.
The spacers i used came with the single speed kit and are a little wider than 8 speed spacing.
This is why i have a thumb shifter on friction mode.
I also put 1/8 chain on so had to widen the rear mech cage with longer bolts and spacers to get the chain to run through the mech.
The small alloy pin in the centre of the mech cage had to come out as well.
The next issue was to get a straight chain line to keep the stress off the gears, i worked out that the cogs had to be shoved right up the outside of the hub to reach desired placing so the mech would not reach the larger gears closer to the end.
So out came the welder and a dual thickness mech hanger was born.
This sorted the problem but raised another as the crap metal had gone soft during welding, so stainless was needed.