Connecting 4 devices on RS232
I am sure I answered essentially the same question a week or so ago, but I can't find it :(

Use an RS232-TTL converter for each device. Now your problem is reduced to combining TTL level signals. The (TTL) TX from your PC can simply be connected to the (TTL) RX lines of the 3 devices.

A TTL-level asynchronous signal is 1 when idle, so you can combine the 3 device (TTL) TX signals using an AND port. Output goes to the the PC's (TTL) RX line.