In AC10 there are different ways of seeing things in 3D. One is just to show everything. The other is to show just what you have selected or in a marquee.
This is really easy to see in the context menu. Just right click on the floor plan (or any window actually!)and see the different options. There is a Show Selection/Marquee in 3D (F5) and Show all in 3D (Ctrl/Cmd+F5). If you have something selected or marqueed, F5 will show that in 3D (just like it did in AC9). Otherwise if you just wanted to see everything in 3D (no matter what is selected/marqueed), then press Ctrl/Cmd+F5).
Furthermore (and this is a bonus), once you are in the 3D window, you can select or marquee elements in there, and again press F5 to see just those elements, and then Ctrl/Cmd+F5 to see everything again. Like a toggle.
Press F2 to get back to the floor plan. F3 will take you back to the 3D window, remembering the state from last time you were in there, either showing just the previous selection/marquee or showing all.
Hope that helps explain the new way in AC10.
Cheers,
Link.