I think that Russia is feeling bad enough about the break up of the Soviet Union and its diminished role in world politics that it is now acting the bully. whenever things such as that happen, we see escalating tensions like those going on now and other countries feel impelled to get involved.
It is possible that the rising tensions between the Ukraine and Russia could lead to a third world war, if other problems come up during this time. However, it is somewhat unlikely. I think the tension would be far worse if the United States gets involved. Thankfully, the United States has stayed out of it thus far, other than throwing words around. If tensions grow in other parts of the world, we could have a problem on our hands.
Russia is not in the best state right now in terms of people siding with them over the situation in Ukraine. In fact, countries are almost unanimously sided with Ukraine (those that are taking notice and care, that is). Ukraine has so many supporters right now that to full on declare war would not end well for Russia at this point. Even with their military.
The speculation that leads to some irrational idea that the conflict in the Ukraine will lead to a third world war is crazy. Now if other countries want to get involved and make a mess of it then maybe something might happen but as I see it people are always looking for something negative to talk about.
The building tensions between Ukraine and Russia could lead to a third world war, but most likely would not. There are always tensions that explode in different parts of the world without it escalating to a full-blown world war. We would hope that diplomatic efforts and perhaps financial compensation would keep the carnage to a minimum.