After upgrading to Windows 10 Pro 64bit I found my Session Drummer not able to be used in X3. The procedure in this article fixed it for me and Session Drummer is back! https://www.cakewalk.com/...recognized-by-SONAR-X3

 
Try this, as I had the same Problem when I updated from x1 to x3.
 
Go To; Insert, Soft Synth, VST2 and you will see in the drop-down:

• Session Drummer 2
• Session Drummer

Click "Session Drummer" which is in fact:  Session Drummer 3
 
I experienced the same thing (I.e. I couldn't find Session Drummer 3) but upon investigating and opening
"Session Drummer", I discovered that it opens Session Drummer 3.
I hope this simple finding works for you as it has for me.
Hey Cakewalk.....Brilliant!!

Open SONAR x3
and Create a New Project.
Go To:
> INSERT
> Soft Synth
>VST2
You should see in the VST2 "Drop-Down" menu:

• Session Drummer 2
• Session Drummer

Open "Session Drummer" and you should see that this is in fact.... Session Drummer 3
 

There is already a fix for this (was released last year).
the comment you are replying to is almost a year old.
It all good though, I think I did the same thing when I joined.
Welcome to the forum.