I think the problem is that they are not used to the newcomer. They are
just protecting their territory from the "intruder". Something of the
kind has happened to a friend of mine: he has had two males (both
neuter) for years and when a new cat (non-sterilized female) arrived she
had to hide from the two brutes all the time. But now, as a couple of
months have passed, she's become brave and has found her place in the
hierarchy. She can even chase one of the males (who is by half as big as
herself) to take a better place or his piece of sausage but keeps a
respectful distance from the other (the bigger and dominant one). So I
think the treatment the male cats give to a newcomer must change in the
course of time. Maybe your friend should try to help the street cat to
establish her relations with the males one by one and not with the two
of them at once, beginning with a gentler one.

tom brought next idea :

Too many cats and there will be fallout.

She doesn't actually live there.

I have a parallel in my 15 cat clowder: Snowball disappears for up to
three weeks at a time - when she comes back, she gets mauled.  When's
she's home most of the time, there is some tension since she hates the
males who attack her, but all in all, things are peaceful enough that she
can sleep, eat, and wander around the house without being bothered.

When I had only a few cats, introducing a new one took a lot of time and
patience: the resident cats were pretty hostile for a long time.  Now,
with a large clowder, the introductions take only a day or so for my cats
to settle down.

Try putting one of the males outside, and letting the other male and the
female learn to get along with each other......Perhaps the different dynamic
will change things.......