Connect only filtered rows to a child table

Create a report in the connected table that has the data you want to show in the drop-down (Up to 3 Fields and filter that report)

Right-click on the field on the Form and Edit Properties of this Form Element. This will bring you to the correct field on the Customize Form screen. Over on the right, instead of choosing the Default (Record Picker) 

Base the Record picker on the report you just created.