If you go with the split-bolt you run the risk of a red tag. Split bolts are listed for 2 conductors only.
Ilsco makes a listed 3 conductor split bolt. The listing limitation of 2 wires is 2 wires of the max size for the bolt- for instance a #2 split bolt has a listed max for 2 #2 conductors. When used with smaller conductors such as #12 the amount allowed is raised.
