> *clocked at 1MHz*

Assuming that you might be interested in anything up to 10 harmonics of 1 MHz, the wavelength of 10 MHz is 30 metres and, the rule of thumb is that anything longer than about one-tenth of a wavelength is going to potentially be problematic.

> *What is the best way to handle this in your opinion?*

Hence, 3 metres isn't really going to pose a problem to either the data down the stub (30 cm) or the original data path (10 cm). If in doubt, connect to the stub via 1 kΩ resistors and use a differential amplifier to monitor the data signal.

Simulation

[![enter image description here][1]][1]

1 MHz output signal (Vout): -

[![enter image description here][2]][2]

I don't think the edge disruptions are significant for RS-422 integrity. If you do something like this: -

[![enter image description here][3]][3]

You'd be hard-pressed to see any degradation on your main RS-422 signal at all. And the monitor signal also looks good but, as expected is attenuated by 10:1.

Remember, my sims have used unbalanced signals and coax so, you'd need to split the 1 kΩ resistor into two 500 Ω resistors for proper RS-422.

  [1]: https://i.sstatic.net/01U2N.png
  [2]: https://i.sstatic.net/2W4Go.png
  [3]: https://i.sstatic.net/VRhvD.png