I had Mixbus 2 but didn't really use it for one reason or another. I just took the opportunity to upgrade to version 3 and am totally lost in how to use what is basically a new DAW. Does anyone know if there is a video course available anywhere or is it best to just dive into the manual? 

Well I looked all over for a manual and couldn't find one. The link to the manual on the Harrison website says "no longer available." The link to the online manual as posted by Harrison on their forum says "no longer available." Not impressed with Harrison as of yet....

Good question.  I loaded up Mixbus3 and clicked on the manual, which took me to the page that you found; it says "not available".  I went to the Harrison forum, and saw that you had already posted that it is missing.  I also posted there asking about the missing manual.  
 
If I find out it is back up, I will post here.

I think there must be a load of people waiting with baited breath for that manual! Hopefully it's just a spot of maintenance or something. 

Have you tried this ? 
http://media.manula.com/user/3491/3491_3551_3707_en_1412200658.pdf 
 
Kenny

There are the old videos on YOUTUBE for the older versions that will get you up and running!!!! I collected them all and have them at hand when working with Mixbus but I have not seen any in depth videos for 3 yet.
For the older videos just search in YouTube for Harrison Mixbus and you should find a lot of them.

And Thanks, Kenny for the link.

Here are some Mixbus 3 Videos on YouTube
 
https://www.youtube.com/r...?search_query=Mixbus+3

Thanks for the Mixbus3 manual Kenny. I'm always loathe to read documentation or watch videos for older versions just in case something has changed. I looked at Harrison's Mixbus videos on YouTube, they have a few brief "up and running" videos but nothing really in depth. I was hoping for something along the lines of an Eli Krantzberg course on Groove3 :)

There are these (Don't know if they're for the current version):
 
https://www.youtube.com/user/HarrisonConsoles/videos
 
https://www.youtube.com/playlist?list=PL46B79118D1D45A86
 
EDIT:  As always Larry is on the ball, and beat me to it.