Sus is simply short for the word ‘suspicious’. Anyone who seems to have the potential to be an impostor is most definitely ‘sus’ and you should let your crew know.
These can be found throughout all of the gameplay maps. They allow Impostors to quickly jump inside and move between a designated set of rooms.
Elec is short for ‘electrical’, which refers to a section on the most popular map, The Skeld.
If you walk past an impostor mid-kill, but didn’t notice and keep going, then you’re throwing.
Third Impostor is used as a subtle dig at someone who is not being a very good crewmate.