Datasource-servlet-for-dynamic-drop-down Based on field authored

How I can populate drop-down values using data source servlet where drop-down values require multiple fields (text field) values authored on same component ?

I have three fields A B C

then I have field D (drop-down fied ) which requires A B C as input to pull all the values 

How can I pass the value A B C to D (datasource servlet)?