Lubos,
You can achieve what you need with a standard window 'fairly' easily, but it involves a bit of SEOing and a patch on plan. (Both of which are a bit nasty) You also can't automatically schedule the windows either as they are actually larger than they appear, but if you just need it to 'look' correct and still act like a window then try the following -
1. Create your window as usual. It can be a normal 4 sided window, although your 3 sided one should still work.
2. Place in the wall as a normal window, but stretch it so it goes outside of the wall (further out than you need) to make sure the unwanted frame is well out of the way. Set the ganging to turn off the thick line at the unwanted end.
3. Create a slab to act as your Operator (in blue on image), so it hits the wall at 45 degrees and extends to completely cover the extent of the window. Adjust in 3d to vertically cover the window completely. Put it on a layer that will be hidden for printing.
4. Use SEO to subtract the slab away from the WALL the window is in (not the window directly - It wont work!)
5. Rinse and repeat for the other half of the corner window all as before.
6. It may be worth grouping the SEO Operator to the wall so it doesn't get lost if the wall moves.
7. Create a patch to make it look lovely on plan. You may need to make it slightly larger then the SEOed area, as the thick lines tend to creep out from underneath which will look wrong. Feel free to add extra hotspots to help align it on plan.
Hope that may be of some help to you.
Image attached is the window after SEOing, and operators still visible.
The hidden operator is in blue. The patch is highlighted in green on plan and moved to one side for clarity.
