Yes, this can absolutely work, at least with certain restrictions / under well defined voltage conditions.keep your wiring for C:TX - A:RX and B:RX.for the other direction, combine the A+B TX data to one signal for the C:RX pin, by using a 'half-duplex' wiring scheme with a diode and a resistor, as for example shown in or in this. Both variants should work ok IMHO, if A and B are well with in the RS232 voltage specs.Limitation: A and B cannot send data at the same time, i.e. 'Half-duplex' only.

Actual Installer 4 2 Serial PortsActual Installer 4 2 Serial Ports

Rs232

Windows xp black edition iso. If A and B send at the same time, you will receive undefined/garbage data.Btw, there are even other. Check the second answer there that received two upvotes.