Should I set same or different channels?
Channels should be optimal for the environment. Difficult to give a more directive answer. Maybe only one channel is really free. But there is absolutly no need to be on the same freq. If more than one channel is usable. Then it is preferred to use different channels.( cfr @Sob) Because if the same channel is used , everything will wait for air-time to transmit. Air-time can only be assigned once in a channel. (Any signal that is decodable (even when not usable for data transmission) will make others wait for air-time.) Client devices have no problem with roaming when different channels are used.
How to include 2.4GHz and 5GHz interfaces from B? Just leave them in same bridge and change SSID?
Leave them on the same bridge. Make all SSID identical between 2.4 and 5 GHz in A, B and C. (*)
Do I need to set same security profiles?
Yes , within the same MT just use the same security profile. Copy the security profile to the other MT.
Is there anything else what I should consider?
Well yes. Within 2.4GHz and within 5 GHz keep everything as similar as possible. E.G the 802.11 protocols used, the channel width.
Your setup is OK for the client devices to maintain their DHCP leased IP address. (They use the same DHCP server). This is essential !
(*) If there is a problem with client devices that flap between 2.4GHz and 5 GHz, then consider using
or adding a specific 5GHz SSID name, that is only used for all the 5 GHz. [adding an extra SSID is by adding a "Virtual Wireless interface" with it's own SSID, extra WLAN is also to be connected to the bridge]
One more thing on client roaming. A client device will roam to any known or saved SSID if it loses contact. This might be very disturbing as that different SSID might be a different network, or require a different IP address. Not flagging "auto connect" for that SSID is not always enough. The only thing to do then is to "forget" the previous connected networks that are possibly in range. (But "open" networks may still be a problem with some clients that connect to whatever they see)