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@allObjects
In your test setup as shown in the picture, I assume you can check with a multi meter the connectivity of all slave RX and master TX, and also the connectivity of all slave TX and master RX. If so, you need only one LED and 220..330 Ohm in series connected from 3.3 to each master RX and master TX to see when something goes on.
Having no resistor what so ever in series with the LEDs and having 3 LEDs on each master RX and master TX lines, the active pin is overloaded and does not guarantee anymore a clear LOW - the LED may still light, though.
That you see the LEDs unequally lit on either master RX and master TX lines indicates that they vary significantly in (forward) voltage drop... and the one with the lowest prevents the other ones not getting the voltage anymore to light 'equally'. Adding resistors already helps... The (forward) voltage drop of LED vary with the color.
As @Gordon points out, for long lines you would need some drivers that can handle noise. You then can add LEDs in series with a resistors to each RX and TX of each Espruino. With suggested RS485 drivers you need then four wires - two twisted pairs - to get the optimal communication.
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