"Michael Baugh" wrote
You're talking about a sewer line.
Not a matter of cutting off a pipe of unknown contents.
The other two have been verified as not carrying sewage.
Perhaps I've misread the OP?
If so, in what way?

Perhaps you have misread the OP. Their waste water runs the middle pipe.
They in noway state the other two pipes are not sewer lines. The other two
pipes very well may be pipes from near by structures (neighbors). Though
this practice would be unheard of in todays building codes, I know it
existed in previous times. It would be cutting pipes of unknown useage.

OP: "I'm having the side sewer replaced in my 50 yr old home (Tacoma, WA).
On either side of the sewer line as it leaves the foundation, are two
other concrete pipes the same diameter, which join the middle pipe and
dump into the sewer main. We tested and all the wastewater from the
house comes out the middle pipe. What are the two other pipes designed
to do? It's impossible to tell what they lead to, upstream."