I configured an ether-channel between S1 and S3.
All the switches are in the same vlan, and PVST+ is enabled on all of switches.
Now, I don't understand why does S2 have one link up connected to each switch ? does it not create a loop by this behavior ?
I tried to shutdowns all interfaces of S2 but It returns to this state.
Can you post the output of "sh ether-cha summary" for each switch?