Currently, Detail iOS only supports connecting 2 iOS devices to record at the same time.
If you want to record with a third iPhone or camera to capture another angle, you could set it up and import that footage after recording, but note that it will not sync up automatically. Here is how to do that:
In the places in your video timeline you want to bring in your third angle from separate footage:
Make sure your third angle video is saved on your phone
Tap layout and change to split screen (you will change this back later)
Tap sources and choose the video from your Photos and pick the part you want to import
If you want to show that angle full screen, tap Layout and the full screen layout