I’m trying to locate the sewer cleanout. I found a box that looks like it contains it but there are 3 separate plugs. Which one should I use?
I also found this which I’m guessing is something else
You are going to have to do some experimenting. Remove the plugs and start running some water, flushing toilets, etc., and observe which way the water's running. If this is multi-family, there could be separate drains for each unit. If you have a building plan it might show up on it. There's not much else we can do from here.
There are often two cleanouts close together on a single sewer line. These enter the underground line in a fitting which directs a probe one direction or the other on the line. At our house there is a double cleanout about 3 ft from the foundation. A probe in the outer one is directed under the slab and a probe in the inner one is directed toward the street.