Hello Bahri
I assume it is this topology that you are talking about:
Essentially, the three routers are connected not as a point to point connection (obviously, since there are more than two interconnected devices), but as a multiple access network. This multiple access network can be interconnected by switches or any number of layer two devices. So in between the three routers there can indeed be a switch or any number of switches interconnecting these routers within a single subnet/network segment.
The switch was left out of the topology for simplicity’s sake…
I hope this has been helpful!
Laz