Ouch!
I have the ES-1, which is a top loading adapter and it can definitely take 3mm slides. The ES-2 and its JJC clone are both side loading adapters, so I don't know what opportunities there are to trick these adapters and/or their slide holders into accepting 3mm slides.
There are Ebay sellers selling the JJC slide holders separately, so it may be possible to get a spare slide holder and modify it to accept the thicker slides.
Thanks, just received it.
Everything looks fine but the slide holder is only 2mm and all my slides are thicker. :-(