3 Combobox linked and have relationship

Hi

I have some problem with my project and I was searching through the internet to find the answer but I got nothing!

So, my scenario is:
I have 3 lists and I want to create 3 comboboxes.
Comboxbox 2 is dependent to combobox 1 and combobox 3 is dependent to combobox2. However I want the values to be shown only once.
But I have some duplicate data and I don't know how to show them once in combobox1?
also, I dont know how to linked 3 comboboxes?

I hope you can help me!