Create Relationship In Views I have three content types that have a text field called PID.  How do I tell views there is a relationship between these content types?
Here are two ways to set up a relationship, depending upon how it is to be applied to content.  


It is not clear how you are suing the PID field, so I will ignore it.  You could create a field with a name like 'relate' and you could enter a term there that identifies a name for a relationship that various content types would share.  You would then set up a View to filter for that field with the specific term you want and Views will retrieve all the content that shares that term.  Further filtering could narrow the selection further if you desire.
You could use a module such as Relation to define a type of relationship and link pairs of content eg. a 'friends' relationship could be used to link two users in a specific type of relationship.