Is it a good idea to do Masters in multiple subjects rather than doing a PhD? Suppose, someone already has a Masters in CS/EE. Say, he wants to work with Robotics.

Is it a good idea to do a 2nd/3rd Masters-by-research in Robotics rather than doing a PhD in CS/EE (concentration Robotics)?

Why or why not?
This depends on your goals, so doesn't have a definite answer. For industrial work, another masters might work, as would some actual work experience. In fact, for some jobs a doctorate might be viewed as a negative: "We want do-ers here, not just think-era". 

But if you want a successful career in academia, then you really need to get a doctorate. You would be very limited there without it. Hard to get a job, hard to advance. 

Examine your goals and then find a path to achieve them. In fact, many paths might be followed to the same goal.