That means that the Port 3/1/1 Is Tagged in a VLAN. You cant untag a Port un More than one VLAN so first you need to remove the Port from the VLAN.
With a show VLAN, you can check which porta are assigned to which vlans, Tagged or untagged.
Go to that VLAN and do "no tag eth 3/1/1" after it is removed. You can add it as untagged un the required VLAN.