I just got a G3, and it has an 11s hub with an included spacer. (see pic) Originally, I took the spacer off, but quickly realized something was wrong with my 10s Dura Ace 7900 cassette as I rode around the block.
After some searching, I learned that the spacer should be used with 8,9 and 10s. My question is, do I also include the 10s spacer (the very thin one) that I have been using with my cassette.
if the your 10-speed Cassette is loose without its "10s" spacer, then include it in the stack alongside the "extra" spacer ...
that is, 'I' would presume that you would still need the "10s" spacer since the included spacer which comes with the hub is merely intended to snug up the Cogs on 8-and-9-speed Cassettes
However, if the 10 speed Cassette's Cogs are somehow snug without it, then put the "extra" spacer in your toolbox.
According to this chart, you need to add the 1.0mm spacer if it is a Shimano 10 speed cassette, no spacer if it is a Sram 10 speed cassette. I had the same issue. Just bought a G3 and installed it with the 11 speed conversion spacer only on a Shimano 10 speed hub. Had some shifting issues. Did my homework and realized I need the Shimano 1mm spacer