How to connect 3 TTL devices together?
Just connecting all TX wires together, and all RX wires together won't work - you'll have all units talking on one connection, and all listening on the other.

RS-232-like serial communication is designed for two-way communications between two devices.  Adding a third severely complicates matters.

If one device can be considered the master, and in control of the other two units, you can connect the TX line of the master directly to the RX lines of the two slaves.  You would need to ensure that the slaves will only speak when addressed by the master, and use a switch or mux to select which slave TX line is connected to the master's RX line.