I know the areas well enough.
My preference would be:
No 1 - VRBO 348773 (but you are using a sofa bed.)
No 2 - Holiday Rentals 78715 - the location is OK, but not quite as good. However it's closer to the Billa supermarket on the Zattere.
No 3 - 436200. That part of the world does not grab me.

And if you've got a lazy hour, here are a couple of long trip reports:
http://www.fodors.com/community/europe/venice-trip-report-warning-im-most-verbose.cfm

or

http://www.fodors.com/community/europe/venice-a-sort-of-trip-report.cfm

But that's just my opinion.

Venice isn't that large that any of these three locations would totally outshine the others.

I think the first apartment is much too small for two moms and two teens. Who gets the fold out? Nice location if you were on a honeymoon.

The second apartment would be my choice, based on the rooms. Two bathrooms would be heavenly. The front windows might be just a little public... I would ask about noise. I think the location is terrific. Very good.

The third is nice too, but I like the location less, and the one bathroom would make me think twice. I would rate that one third in terms of location.

Have you checked on franco's thread avout accomodations in Venice?

How do I get to Franco's thread?

I'd take the second one. I like that location a lot, and it's a big apartment.