Just buy the three day vaporetto pass. Take the vaporetto ride down the grand canal from P. Roma to San Marco and then just wander around.
If you want to do the Doges secret itineraries tour, you should buy that in advance, otherwise you can chance the line by buying the general admission to the Doges palace when you get there.
You can chance the line for St. Marks if you just want to walk around inside without a tour. But if you want a guided tour, buy that in advance.
Consider the free (pay a tip at the end) walking tour.
http://venicefreewalkingtour.com
I'd do the walking tour first, because the information they give about Venice and where to eat and what to see is useful before you start walking around on your own.
With three days there, you could spend a day at Burano and Murano, using your vaporetto pass to get there.
As for the Venice City Pass, add up the costs of the museums you are going to visit in the three days you are there and compare that to the price of the Pass. Wasn't worth it for us, because for us, Venice is about eye-candy, not the inside of museums.