href="https://www.cpdl.org/wiki/index.php/Secunda_anthologia_vocalis_(Oreste_Ravanello)">Ravenello’s a good starting point. Not many general Eucharistic motets are there but you have an Ave Verum plus some things to sing for the Tantum. And you can reuse all of this at other times of the year. (I would not sing something from the festal liturgy like Ecce panis Angelorum.)

Also there are some three-part Ubi caritas settings. @Drake has an SAA one and I bet that it can be made SSA if the first A range isn’t right for the mezzo-sopranos who can sometimes sing alto.