Not currently, but the hardware potentially could feed 3 devices simultaneously.
There are 3 HVS (Hardware Video Scalar) channels, which each feed into one of the 5 pixel valves at a time, and from there on to output devices. There are a couple of restrictions (eg DSI0 and DPI share a pixel valve), but routing of HVS channel to PV is pretty flexible.
History has only ever required driving 2 displays simultaneously, and so the third HVS channel is assigned in the software to the transposer - it writes back to memory and is used should you wish to transpose (rotate) the display, and also currently for some of the composition with VLC.
If you don't need the transposer then HVS channel 2 would be available and could be used for a 3rd display.
HOWEVER, I see no need to do that in the firmware driver. A full DRM/KMS driver is in the pipeline for Pi4, and with the configuration of that through device tree I can see the possibility of exposing 3 displays.
Memory bandwidth and the V3D max composition size of 7680x7680 are going to limit what is practical, but it's far to early to say where those limits will sit.
Software Engineer at Raspberry Pi Trading. Views expressed are still personal views.
I'm not interested in doing contracts for bespoke functionality - please don't ask.