Installing triplex doorbell, 4wires, help with wiring??

Can someone please help… I am trying to replace a doorbell in a
triplex I live in. Three separate bells, three separate buttons.

There is one 4wire wire (red, green, yellow, black) coming out of the
wall to the door bell.

Doorbell 1
Screw 1: Red
Screw 2: Green

Doorbell 2
Screw 1: Red, Red
Screw 2: Black, Yellow

Doorbell 3 ??? (Doesn't work)
Screw 1: Red
Screw 2: ???? Nothing ????
Doorbell 3 is the problem. When I looked at the doorbell, it appeared
that only one red wire was attached to one of the screws. The other
screw didn't have any wire attached. I could be wrong, but the red
wire on Screw 1 looked like it was one of the red wires from doorbell2
on screw 1??? It came loose when I was uninstalling it, so I don't
know. For that matter, it is possible that a wire on screw 2 fell
behind the wall somehow? I really don't know….

Other information: There is a twist on wire connector (marrette) for
the red wire. Currently there is just one red wire coming out of the
marrette, but that doesn't make sense to me. I have to assume that the
marrette was used to split a red, but I can't see it. I suppose
someone could have just been trying to lengthen the red wire, but that
explanation seems somewhat implausible?

Can anyone who understands electricity or doorbells tell me how door
bell 3 should be connected? Thank you!!
-Rob