Espruino with Glediator & LED matrix

I'm not quite sure I understand. You'd expect Hello World to appear on the terminal program connected to your USB-TTL - but it doesn't? Have you checked that the baud rate of the connected terminal program is set to the same baud rate you set for Serial3?

Or you've left C10/C11 completely disconnected and you were expecting things to be echoed back?

It might be worth trying:

Serial3.on('data', function (data) { console.log("<Serial3> "+JSON.stringify(data)); });

That'll make sure that if there are nonprintable characters, they get escaped so you can see them.

Are you using an up to date (1v81) firmware? Some of the older ones had issues with Serial3/4.