Video switch/streaming which combines three sources into a 3-way split

[Skjalg]

I am looking for a video switcher that can combine three HDMI inputs that are scalable, and format the inputs into a combined output that looks something like the figure below where S2 comes from the section of the computer, S3 from a section of the camera and S1 from another camera. L1 and L2 are phono jack line in and M1 is a microphone with phantom power for voice. The placements of Lx and Mx should be on the video switcher preferably.


[Attachment 52500 - Click to enlarge]

I had a look at ATEM Mini and Roland VR-1HD, but from what I have understood so far they can only do one PiP, not a three-way split.

Have I overlooked something on the two devices with regards to split screen? What other options should I have a look at. I would prefer an all in one box.
My intention would be to live stream it via Zoom, or possibly Messenger. My computer is a 2018 Mac Book Pro, 15 inch, if that is relevant.