Hi,

Think of it as 3 cascading drop down lists. Instead of populating the 3rd drop down, you can filter the content of the drop down based on first 2. What ever the value you want to populate in 3rd drop down, have it already populated and then filter.

Cascading drop downs and Nintex Forms for SharePoint​

Here is 3 tier drop downs: Three-Tier Cascading Drop Downs: Do Go Chasing Waterfalls!  and associated post: Three levels cascading lookup? . Hope this helps.

I agree with Kapil on this one. Seems that should get you what you need by just cascading them.

Thanks Kapil it's really helpful