It's up to you how you want to organize buildings among tiles. If the buildings have simple geometry it is usually a good idea to combine multiple into a single b3dm tile. Otherwise if the buildings are complex, they can split up into multiple tiles. In the end its less about buildings and more about geometry and how you wish to subdivide it.
There aren't too many publicly available tools that will convert source data into 3d-tiles yet. However we are working on some internal tools and would be glad to test out your data if you would like. You can contact me at
sli...@agi.com.
Let us know if you have more 3D Tiles questions.