Dependent Data Validation

I'm attempting to create a form that uses dependent data validation to narrow down the results in each subsequent drop down list. I've perused through other threads on this topic, but haven't been able to figure out exactly how to set up the sheet to acheive what I want to do. It seems like it ought to be straight forward.

The objective is this: I need to have "three tiered" data validaiton form that can ignore duplicate entries. In the example I've attached, my goal is to be able to have a user first select a Site > Then select a Phase (while ignoring duplicate Phases under the same Site) > Then an Activity. Once the activity is selected, I'll use a VLOOKUP function to return the Activity ID number elsewhere on the sheet. Ultimately my list will have many hundreds of entries and will need to be amendable. That is to say, I'll be adding new sites, activities, descriptions, and ID numbers over time. I'd like the form to be able to recognize those additional entries and incorporate them into the Data Validaiton lists.

I've been able to read through tutorials here: http://www.contextures.com/xldataval01.html which have helped me get the basics. I'm currently hung up on the second validation list. I can choose a site, but once the site is chosen, I can't eliminate the duplicate entries in the list of Phases. (e.g. If I select Site = MPS, my dropdown list for Phase will include "Preparation" twice. I'm also at a loss for making the data list amendable.

Any help would be greatly appreciated! Thanks. Example.jpg