HOW DO I RECORD A V-DRUM KIT USING MIDI IN SONAR?

I have now wasted 3 days trying to get this to work!
Can anyone point me to some good guidelines on how to simply record a V-drum kit midi track into Sonar.
I have a TD-3 kit, UM-ONE MIDI to USB cable and Sonar Artist with Session Drummer.
WHY IS IT SO COMPLICATED??!?!
(The only time I did manage to get something recorded the playback was terrible - out of time, hanging, slowing down, etc..)
And please don't point me to the Cakewalk Help guide, it's useless.

It's not going to be as easy as it might appear if you are going to trigger Session Drummer for your drum sounds  via MIDI.
 
You will need to learn how to configure your PC and Sonar for use with virtual instruments, and to get the real-time audio latency as low as possible. By default, your Windows audio drivers are not going to get the job done.
 
Your general PC setup, as well as audio and MIDI setup will be crucial for getting usable real-time audio performance with your MIDI drums (or any virtual instrument).  See this PC optimization guide: https://www.sweetwater.co...-guide-for-windows-10/
 
The Cakewalk help guides are useful for getting over those first steps with setting up MIDI and audio in Sonar.

What you are trying to do is pretty complicated, so best not to complain about how complicated it is.
 
I have done this numerous times, so it is not impossible.  Now if I understand you correctly, you want to play the V-drum kit, and capture the midi output of the V-drums onto a track in Sonar: is that correct?
 
1) take a deep breath, try to calm yourself.  Remember that you are doing something pretty complicated and a calm frame of mind is needed.
2) Connect the midi out of the V-drum brain to the midi in of your midi interface.
3) Launch Sonar and create a new project.  Add a midi track to your new project.  Arm the midi track for recording. 
4) You need to tell the midi track to receive midi data from your midi interface, which midi port the data will be on, and which midi channel the data will be on.  I use the Track View for these chores.
4) Check the metronome and set the number of count-in measures to 0.  (How you set up the rest of the metronome depends on whether you plan to play drums to a metronome: you may also want to adjust the project tempo to whatever tempo you will be playing at - depending on whether you want the midi data to fall on the MBT timeline correctly.)
5) Set your V-drum brain to send the midi data on the correct midi channel (i.e. the midi channel you instructed Sonar to receive midi data from).
6) Press the record button on Sonar.  It should immediately be recording.
7) Play a couple of hits on your V-drums.  Midi data should now appear on the midi track you created in Sonar.
8) Report back to us what happened when you tried this procedure.  Report back to us if you do not understand any of the above instructions.
 
Don't forget to have fun.

Oh boy, been there done that.
 To make a long story short you need a MIDISPORT 2X2  or if you want to go larger the MIDIPSORT 4X4 between your V Drums and computer, and problems solved. These are NOT audio devices! They are dedicated MIDI patch bays, that's all they do, and they share their USB connection with nothing else but the devices that are plugged into it.
 Works great has super fast communication/transmission  with all Roland, Alesis, Yahama electronic drums, or anything else with MIDI control I've ever plugged into it
 I've been using big brother 8x8 for 2 decades never has any MIDI problems since, and never even had to update the drivers.
many years ago M-Audio bought out original manufacture MIDIMAN and never tried to improve change anything but the color and graphics.
 

Steev
Oh boy, been there done that.
 To make a long story short you need a MIDISPORT 2X2  or if you want to go larger the MIDIPSORT 4X4 between your V Drums and computer, and problems solved. These are NOT audio devices! They are dedicated MIDI patch bays, that's all they do, and they share their USB connection with nothing else but the devices that are plugged into it.
 Works great has super fast communication/transmission  with all Roland, Alesis, Yahama electronic drums, or anything else with MIDI control I've ever plugged into it
 I've been using big brother 8x8 for 2 decades never has any MIDI problems since, and never even had to update the drivers.
many years ago M-Audio bought out original manufacture MIDIMAN and never tried to improve change anything but the color and graphics.
 

For years, I would have recommended the MIDISPORT range. I'm using the original 8x8 and the 2x2 anniversary edition (actually two of these).
 
Since the 1803 release however, the 2x2 has been giving me dropouts - disconnecting randomly and generally being unreliable when MIDI traffic is high. I've got a dual boot and if I switch to my Windows 7 boot, there's no issues.
 
I've since moved to a MIDITech MidiFace 8x8: https://www.thomann.de/gb/miditech_midiface_8x8.htm
 
This has been absolutely rock solid for me.
 
I still use my MIDISPORT 8x8 for all my rack MIDI synths, but these get very light use nowadays.

Have you made sure the TD3 head is set to "Local Control Off"? 1. Hold down [INST] and turn the TD-3’s power on. “Local Control oFF” appears in the scroll display, and Local Control is switched off. Hitting the pads creates no sound from the TD-3. (the trigger indicators do light up.)
 
I use a TD3 with CbB but I'm using BFD2 not Session Drummer.  I can't help you there...

If you don't have an audio interface designed for recording and/or a slow computer (by the way, what are your computer specs and audio interface?), don't monitor from the computer and don't use soft synths and effects.
I monitor my kit audibly and the computer sounds audibly while playing and recording. I plug both my drum kit audio, such as it is, and my interface audio into a small mixer and monitor from that, either through headphones or a stereo receiver if the room mate isn't home. 
Only after recording do I add synths and effects.

 
I didn't realize this is a "vintage" device.  Yes you are wise to invest in a proper MIDI interface. Those cables are garbage from what I've read here. 
If your planning on recording Audio you will need an Interface anyhow, just make sure the interface you buy has MIDI ports and all will be good.