Milan / Lake Como to Venice is easy by train. Look up the "seat61" website for info about trains anywhere in the world, including Italy.
However, Plitviče lakes are hard to reach. Also, I don't think you need more than two nights there. Even one would be enough, and a day trip is feasible - if crowded.
With that in mind, I would perhaps take the ferry from Venice to Pula (or Rovinj), in Istria, maybe stay there 1-2 nights depending on the time you have, and rent a car there to drive to Plitviče lakes, then to Split.
Or, more comfortably, travel by train from Venice to Ancona and take the overnight ferry to Split. Base yourselves there, and take a day trip or an overnight trip to the lakes.
Last suggestion is to change the order of your trip. Could you land in Venice, if your flights aren't booked?
Then you can fly from Bergamo (easily reached from lake Como) to Zagreb, from where reaching Plitviče lakes won't be as hard.
All in all: it's complicated. Maybe someone has better ideas than me.