Mohammad:
You can probably see the RS485 bus work fine with three devices, each with its own terminations as long as you keep the cables short and do not try to use the fastest baud rates.

The actual Rs485 bus as you know realistically wants to have the nominal terminator resistor on just the extreme ends of the bus. The RS485 driver that is currently actively driving the bus, when there are two terminations, is having to source and sink current into the two 133 ohm resistors in parallel. This is an equivalent load of about 62 ohms on the driver.

When you add the third device the equivalent load drops to about 40 ohms. At +/- 5 volt signal swings this is asking the driver to source and sink 125 mA. This is beginning to get kind of hard on the drivers. Think what would happen if you went even further and had 10 devices all with terminators. Then the equivalent load would be 12 or 13 ohms and the drivers would probably fry themselves trying to source and sink over 400 mA.

There is an additional issue of having the extra resistors that causes a problem. Especially at high baud rates and / or long cables. The extra load on the driver tends to decrease the voltage swing that the driver can produce. At just two resistors the driver can easily get to +/- 4.8 volt swing, but at loads that demand more current from driver the available swing will decrease. This has the effect of increasing the sensitivity of the receiver devices on the bus to being more sensitive to noise the closer and closer the reduced driver swing gets to the threshold voltage of the reveiver.

The last thing I would like to add is that when the termination resistors are "permanently" installed in devices like you are working with, one solution is a pair of wire cutters. Then the resistors can be installed in the field wiring cabling instead. You are facing one of the problems of dumb equipment designers whom do not provide a jumper or switch option to open up and disable the termination on any particular piece of equipment.

I have seen a rare instance however when a piece of RS-485 equipment used internal active components and/or a relay to permit the termination to be switched in and out from some kind of user interface configuration toggle. Maybe one of your pieces of equipment has this feature.

Good Luck
Mike Karas