One of the many manuscripts that are still available to us is the Salzinnes Antiphonal. Not only is this manuscript beautifully scanned but also has a menu tab that gives the text in easy to read modern fonts.

For Passion Sunday the following Hymns are found therein,

Vespers and Matins Hymn,  Hymnum dicamus domino laudes

Compline Hymn, Crux fidelis inter omnes

Terce Hymn,  Vexilla regis prodeunt fulget

N.B. The Lauds Hymn uses verse 5 onwards of the Hymnum dicamus domino laudes

Other Passiontide Hymns can be found here, Passiontide