Look at the side of the receptacle and see if the tab between the screws is broken off. This tells you if it is one circuit or two. It could also be a "half hot" receptacle if the tab is broken off.
You are not allowed to connect 3 wires to a receptacle. There are 2 terminals on each side so only 2 wires can be connected to each side. One wire goes to each screw maximum. Never use the push ins as they make a bad connection and are subject to failure as well as not being rated for 20 amp circuits. In a case like this you will need to pigtail the wires with wirenuts.
The old receptacle had all 3 sets of hot/neutral wires plugged into the back of the receptacle. I can't find any bridges like I'd have expected on a screw terminal receptacle, which is what I'd hoped to find.
Is there a reason why there are 6 hot/neutral wires and only one ground?
I knew the push ins were not a good idea and the one I'm looking at is looking kind of scorched, too.
How do I know which wires to pigtail, i.e. which are from the incoming circuits and which are to the downstream circuits?