Splicing 6/3 wire to multiple 8/3 wires (or even other 6/3 wires) inside a junction box
I use Polaris and other brands for this, in the past I used to use cambric , linerless and super88 to insulate the split bolts the cost of the Polaris style pays the first time you reuse them and are much easier to use. I use a size 4-14 rated alcu  Home Depot actually had a good price on these and I usually put tape on to hold the plugs in place but nothing is required to hold them in the box.  Their are 2 lug 3 that I think you want up to 6 I have purchased all 600v rated and UL approved.