This is a proper overlay comparison... click on pix to enlarge.
View attachment 389995
All things being equal, I perfectly aligned the diameter of the front 20" wheel on both the Model-P3 and Model-PY cars. That's why the front wheel looks sharp in the pix even though it's made up of 2 overlapping pictures. Therefore all the other dimensions should be pretty close. (I used Tesla's renderings, which are normally generated directly off their CAD system, so they're accurate to each other.)
The overlap clearly shows the center of the rear wheel on the Y extending further to the rear than the 3 by approx. 2"-3" according to my software. How else are they going to get that third row of seats in the Y and still have cargo space? When entered into my software, the Y has a longer wheelbase and most likely has a wider track as well.
They may share the same platform, but that doesn't necessarily mean they share the same wheelbase & track dimensions. No one will know for sure until Tesla releases the full specs.