There are couple things to consider, for example there are plenty military bases in Russia, with weapons and ammunition storage, there need be some military that protect them, those can't be sent to the front. Similarly there are soldiers that need to protect the whole borders, Russia is a huge country so there are soldiers in garnison in the east for example, near north korea.
Out of the 3 million you also have the "reserve" which is just people that did the military service once when they were younger. That represent 2 millions, those are not full time soldier and there is a very high political cost to mobilize them, especially if you deny the war and call it 'special operation'.
And in the 1 million left, you count all the military personnel, including many people who are working for the military but are not directly fighters, like people who organize the supply chain and the planes maintenance and things like that. Those are already active during the war, even though not counted toward the 200K. The 200K represent an estimation based on satellited images with vehicules, trains, tents , barracks and so on being visible it gave an estimation.
Those guys seem to have done the maths : https://www.youtube.com/watch?v=b4wRdoWpw0w ( 20 min video from 5th of March 2022 )