Where could I find the antiphons and Psalms/Canticle in Latin for the current celebration of Vespers I of 3rd Sunday of Lent? I have parts, but not all. Also, is the reading available in notation to be chanted?
However, Antiphon 1 and the Magnificat antiphon differ from the Latin text I have: Ant. 1: Pænitémini et crédite Evangélio, dicit Dóminus. Mag. Ant.: Dicit Dóminus: Si quis bíberit aquam, quam ego dabo ei, non sítiet in ætérnum.
The long answer to your question could fill libraries.
The short answer is the the composers of the Liturgia Horarum used antiphons for which no chant ever existed, and so the official chant books had to draw other, similar texts from the existing chant corpus.
I asked this question myself in another forum a few weeks ago. The Office is very, very different between the old and new rites. You will want to use chants prescribed by the Ordo Cantus Officii. Unfortunately, the official book does not include the actual chants, but only references where to find them.
However, the internet is an amazing place. Someone posted in the forum last year a document he had compiled himself with the OCO AND all the antiphon chants! I don’t know yet how to link things here in the forum (will someone link it here?). Also, you can go to antiphonale.net, which lists all the appropriate antiphons, and is linked to Gregobase.
Here's the most recent version of the OCO with melodies. Thanks to diligent proof reading by members of this forum it's been possible to eliminate many errors found in a previous version.
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