Conditional formula

Bear with me: Im coming from filmmaker and Im not an IT person, just a lawyer who likes to make my own “apps”.
I think I understand the theory of the conditional formula but I want to have a field which uses the data from one of 3 fields: DX, PO Box, Street Address.
Not everyone has a DX - if for a given record there is no DX then I want to use the PO Box and if there is no PO Box then street address will do.
The formula I used in filemaker had the structure - If(Dx is not empty, use DX,(If PO Box is not empty, use PO Box, Street Address)). The brackets may be slightly misplaced.
What I cannot find for Ragic is the syntax for “not empty”


Not empty is written as !=""
Here is our documentation of formulas in Ragic:

Look at the IFS() formula for what you need.

I did - I still can’t see where it says anything about “not empty”
Thanks anyway.
I went the other way - I made the condition based on emptiness.