The three are interchangeable !
The first one means

I’m saying that this is disturbing, but I could have said that this is frightening

the second means

if this is not frightening, this is disturbing

and the last one means

if this is not disturbing, this is frightening.

As a french person I use the second one more ! But an other common way to say this sentence is:

Ce qu’ils ont découvert est troublant, voire effrayant.

Which means “I’m saying that this is disturbing, I could have said that this is frightening but it’s maybe too much”, it’s not exactly the same meaning but we often use it to say “X, if not Y”