The LHS here has packages of wire with three wires twisted...
Most of the time I just cut the wires some where in the middle, between the plug and the servo, cut the wire the lenght needed plus an inch or two and solder everything together...heat shrink needs to be put on a ways back on the wire before soldering...
Expert makes a decent priced Y harnes that you can easily lengethen any way ya want it..
Buying the ends and the tool to crimp then are a pain...
Easier and usually cheeper to start out with something already made and 'customise it' to your needs...

Just two cents worth...