OptiPlex 7080, three monitors, two DisplayPorts

If you have only the 2 DP ports, then I think you're limited to two monitors, unless you get a video card that has at least 3 outputs.  Once you install the video card, the system bios will switch from the internal video to that card, and you should be able to use 3 monitors, once you install the drivers for that video card.  

You can add graphic card but in this case you have optional slot in motherboard you can insert VGA socket to run three monitors by internal graphic card without adding external graphic card 

You can add HDMI connector for your third monitor. 

Yes it's possible, you can google for "monitor daisy chain" but it would require at least one monitor supporting relevant DP pass-through signal or additional DP splitter ("extended" option, not simple clone/duplicate). Other options obviously include adding dongle (singe 7080 does support this with Dell's proprietary module with either HDMI/DP/VGA/USB-C ports) or via additional GPU (e.g. via PCIe x16 slot).

OP has not explicitly said the form factor of 7080.  The pic of OP discussion is for 7080 MT.   MT, SFF, Micro all have 2 integrated DP.    They all have a slot w dummy cover for optional third video port: VGA/HDMI2.0/DP++1.4/Type-C w/DP-Alt mode

If OP has three identical monitors using DP, one option is add a third Dell DP port.  This connects to motherboard video socket as a third integrated DP.

For SFF and MFF

YPF8G

For MT