That aaaahhh sound is gorgeous and crops up in many great places across the ABBA catalogue. I love the work it does in The Day Before You Came. It might not be part of the third voice because, to my ears at least, the texture seems to lean a little more on Frida - even though it's clearly both voices. Maybe it's to do with the key of the songs.
ALSO!! How could I have failed to mention those gorgeously angelic call-and-repeat backing vocals in Should I Laugh Or Cry? Agnetha is right up in the stratosphere. I love that naggingly repetitive guitar that underscores the song too.
I think Lovelight reveals a different, fascinating and lovely deployment of Agnetha and Frida's voices. In the first half of the song's verses, it sounds to me like it's Agnetha harmonising below her own already low lead; with Frida then joining in the second half of these verses with her beautiful harmonising.
As wonderfully as their voices were used, overall, in ABBA, I think Agnetha and Frida's vocals could have been switched around a bit more - as when Frida harmonised above Agnetha in the third verse of Move On, as Michal mentioned.