Is there a tool designed for holding multiple electrical wires together to keep them in place for wire nut?
Yes. It's called Lineman's Pliers.



To use:


Strip the wires a bit longer than required (1" - 1 1/2").
Hold the wires at the insulation between your thumb and forefinger.
Pinch the tip of all the wires in the lineman's pliers. 
Twist the wires together using the pliers.
Trim the end of the wires to the proper length.
Twist on the twist-on wire connector.






You could also try using crimp connectors, with insulated caps.



But then you'll need a crimp tool





Or as auujay points out, you can use push-in wire connectors. Which you can read about on the blog