In each case they did not fight an army. Top Russian veterans on the ground say that their regular ground forces literally got negative experience out of the Syrian war. They were not attacked and learned to be even more pathetic and lazy. Only the Russian airforce got experience, and that experience was in bombing civilians and blowing up cities.
In South Ossetia, like Ukraine when the Russians first invasion, the Georgian government had been under the control of a series if Pro-Russian puppets who had deliberately ransacked the Georgian army. In the first invasion on Ukraine (well, it's really just been one long invasion, but you know what I mean), this was the same problem. Yet look what happened 8 years later: Ukraine's army suddenly became a real army, and we see videos of a single tank (on more than one occasion!) taking out an entire Russian column that behaves like it has never received even basic training. The infantry just scatter in terror and the Russian tanks just start firing into buildings at random, not knowing from where they are being killed from. In Chechnya, the population is far smaller than that of Ukraine, and there was no military to speak of. Resistance consisted of rag tags with no artillery, no anti-air, no anti-tank capability. Nevertheless, the Russkies couldn't take them on fair. They had to destroy entire cities resulting in the deaths of 150,000-250,000 civilians. So to sum it up: Russkies just aren't good at war.