I would say that Cities Skylines is a VERY GOOD replacement for Cities in Motion with the fact that, as previously mentioned, the Mass Transit DLC and many mods make for VERY powerful public transit simulation. As for the missing Rush Hour, there is a mod for that (I think it is actually called rush hour).
However, Skylines is an AWEFUL replacement due to the fact that it is such a demanding game and it performs like crap unless you have a really good gaming PC. I have a gaming PC (lower-ish end) and with no mods, I still can no longer run the game in a playable state, and as mentioned previously, mods are very much needed for Skylines to replicate Cities in Motion.
But, as usual, that is an example of society leaving behind those who cannot afford to upgrade. I am not upset about that fact, it is just how it works, which unfortunately means that a Cities in Motion 3 will ever come out. It is like Rockstar's Midnight Club. You wont be seeing any more of those because the "Race Mode" of Grand Theft Auto has replaced it.
So, to answer the Original Question: I don't think a Cities in Motion 3 will ever be released, and Cities: Skylines can be a "replacement" but only if you have a good enough computer for an overly-complicated game for a rather simple feature that is Cities in Motion.