<< Am I mistaken, or is his 3 volume set simply an abridged version of his 21 volume set? >>
As far as I know, Jurgens is a 3-volume set by itself, not an abridgement of anything. Although Jurgens may have edited larger sets of the Fathers.
The Jurgens 3-volume set contains most or all of the “classic” passages from the Fathers, arranged at the back in an index by particular doctrine or topic. Of course it is a matter of interpretation just how much any particular passage included by Jurgens supports any particular doctrine.
It is enough to show the early Fathers were clearly Catholic in theology (on Baptism, Eucharist, Penance, sacraments, hierarchy, bishops, priesthood, primacy of Rome and papacy, Marian doctrines, Scripture and Tradition, moral teachings, etc).
It is not enough if you want to know everything a particular Father said about a particular doctrine. For that, specialized books on a particular Father (e.g. JND Kelly’s books on St. Jerome or Golden Mouth: St. John Chrysostom), or specialized books on particular doctrines (such as History of the Doctrine of the Holy Eucharist by Stone, or A History of Penance by Watkins, or Mariology by Juniper Carol) or a more complete set of the Fathers such as the 38-volumes from Eerdmans (edited by Protestant scholars) or Catholic sets (such as provided by NewAdvent.org).
Another one-volume selection of quotations from the Fathers similar to Jurgens but obviously smaller in coverage is The Teachings of the Church Fathers edited by Willis, also available from Ignatius Press. Again, adequate to show the early Fathers were clearly Catholic, not adequate if you want everything a particular Father said on a particular doctrine.
But for most of the debates you will have in here, Jurgens and Willis is normally enough.