Asset Manager

hi,
I am trying to create a simple asset manager using 3 content manager databases.

From this, I want to create a dynamic drop down box displaying all usernames, when selected, it will filter the tables below with the information of which they are the owner.

Users - Username(Primary Key), Email, First Name
Assets - Owner (linked to username) (can me more than one owner to an asset) , Manufacturer, Model, Acquired Date, Description, Condition, Image, etc…
Websites - Owner (linked to Username), username, password

I can’t seem to get the filtering and relationships to work…
any suggestions.