You can Navajo ply from a single bobbin, which gives a three ply yarn - there's bound to be a Utube 'how to' on Navajo plying; it looks complicated but once you get a rhythm going it's fun.
If your other singles are fairly similar in wpi (wraps per inch) to your Corriedale you can ply them together, but stop and have a think what you will use the yarn for before you do it.