I just got Windows Media Center edition up and running with 3 tuners. I am using one old standard edition Hauppauge PVR-250 and one new Hauppauge PVR-500MCE. The PVR-500 has two tuners, so together with the 250 I have three.

It took some experimenting to get this up and running, so I’ll post some of my experiences here.

With three tuners, this is how the registry and your Device Manager should look like:

This is how I did it.
1. Install both cards
2. Install MCE and drivers for the PVR-500 (NOT drivers for the 250)
3. Start MCE and set up the PVR-500 so that you have two tuners up and running
4. Then I tried to follow the instructions here:
Howto: Setup 3 analog Tuners (Pinned)

I encountered some differences, and will try to describe them here:

Close MCE
Start regedit
Open [HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionMedia Center]
Export the Media Center Key and all Subkeys, save it as Backup.reg (This will help you with restoring the settings you started with if something doesnt work.

Open [HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionMedia CenterServiceVideoTuners]
Within that key is another key named DVR and another named something like {A799A800-A46D-11D0-A18C-00A02401DCD4}
It will look like this

-Tuners
-{A799A800-A46D-11D0-A18C-00A02401DCD4}
-{04CBCD6F-0EFE-4E52-9209-43B6AC9D4217}
+TuneRequest
+UserSettings
-{54B4E960-C8E5-42A2-AC64-98A586B7BEA2}
+TuneRequest
+UserSettings
+DVR

Here, I actually had three groups of this type:
-{54B4E960-C8E5-42A2-AC64-98A586B7BEA2}
+TuneRequest
+UserSettings

I exported all three.

Delete both of those keys and it will look like this.
….I deleted all three.

-Tuners
-{A799A800-A46D-11D0-A18C-00A02401DCD4}
+DVR

Close Regedit and Open the device manager
Open sound, video and game controllers, Disable Your TV cards

Shut down, and install your new capture card, then restart. If your card is already installed Reboot anyway.
….here I installed the drivers for my 250 card.

Start MCE, and go to settings > TV > Setup TV Signal
Run through the setup process for your tuner, and guide.
Make sure it works and is not stuttering.

Close MCE

Open Regedit, and you should see this.

-Tuners
-{A799A800-A46D-11D0-A18C-00A02401DCD4}
-{BAF147BF-2181-466D-96FE-48020D0861DD}
+TuneRequest
+UserSettings
+DVR

Expand the keys make sure the keys are filled out.

-Tuners
-{A799A800-A46D-11D0-A18C-00A02401DCD4}
-{BAF147BF-2181-466D-96FE-48020D0861DD}
+TuneRequest
Components
Tuning Space
+UserSettings
Components
Tuning Space
+DVR

Export the key {BAF147BF-2181-466D-96FE-48020D0861DD} save it as new card.
Restart Your computer now.

Open Device Manager
Open sound, video and game controllers, Enable Your TV cards

Import the Registry Entries you exported
Double click on both top.reg and bottom.reg and say yes to import them.
Reboot
….I imported all three of them.

Open Regedit, make sure your on these keys

-Tuners
-{A799A800-A46D-11D0-A18C-00A02401DCD4}
-{04CBCD6F-0EFE-4E52-9209-43B6AC9D4217}
+TuneRequest
+UserSettings
-{54B4E960-C8E5-42A2-AC64-98A586B7BEA2}
+TuneRequest
+UserSettings
-{BAF147BF-2181-466D-96FE-48020D0861DD}
+TuneRequest
+UserSettings
+DVR

Here, I had lots of entries. It looked like I had about seven tuners(!)
….and MCE only played with ONE tuner.

Some of the entries missed some "folders", I deleted them, but I still had four main enries. Looking like I had four tuners. I rebooted and tried again after I had deleted the entries that was not complete. Now MCE played with TWO tuners. This is how a complete entry should look like:

I examined the four entries and discovered that I had one entry with DevInstance 1, one with DevInstance 2 and TWO with DevInstance 0. Click the thumbnail to see where you find DevInstance:

I deleted one of the entries with DevInstance 0 so that I had only three entries. One with DevInstance 0, one 1 and one 2.

Now MCE plays with 3 tuners!