Hey,
I know what you mean, I can't live without TodayAgenda either. You can have TodayAgenda and TF3D, just not at the same time. Here's how:
Click
here to download SecondToday and install it. It will create a second Today screen for you in which you can enable any plug-ins that you want.
After you've installed SecondToday, launch it and enable TodayAgenda (assuming you already have it installed). Tweak the TodayAgenda settings and you're good to go.
Now, how to access it?
The easy way is to simply make SecondToday one of the apps in your TF3D Programs tab.
I like to be able to access it directly from my Home screen, so I've mapped SecondToday to the left softkey in my TF3D Home tab. Here's the keys to edit (not my fault if you manage to break your registry).
HKLM\Software\HTC\Manila
Change HomeLSKPath to: \Program Files\SecondToday\SecondToday.exe
Change HomeLSKText to: SecondToday
That should do it!