you will get Gigabit speed across all devices which are connected directly to switch because they are directly connected to each other based on MAC address but any traffic which has to go through router will be limited to 100 Mbps.
Device B and C will have Gigabit speed
Device A and C OR A and B will be limited to 100 Mbps
Device B and C will have Gigabit speed
Device A and C OR A and B will be limited to 100 Mbps