Architecture Suggestions Desired

I have a case where three different types of posts share some common fields, I started to go down the road of creating a model that has all of the fields merged together and then make a bulk of them nullable for when the post type doesn't need them.

It began to feel a bit wrong to do it this way but my initial thought was for easier searching. So my question is basically do I forge ahead on my current path, or trust my gut feelings and tailor the searching across multiple tables?

Second part, if I keep going on this current path, can I get creative and use the same form but selectively hide fields that do not apply to a given post type?