The 3 volume edition was originally translated from the Latin a year earlier than the ICEL version. The translations were copyrighted to the heirachies of England & Wales, Ireland, and Australia.
The volumes are
I- Advent, Christmas, Weeks 1–9 of Ordinary Time
II -Lent and Easter
III- rest of Ordinary Time
The contents are the same insofar as they are translated form the same Latin books, use the same sequence of psalms, and used in the same liturgical year with the same saints, etc. However the translations are different, For e.g. Thursday Psalm Week 4
UK (i.e. 3 volume)
The prescribed hymns are also different since neither translates the hymns of the original Latin edition entirely.
The psalm version used is the same- the Grail psalms of 1963. There are 3 sources of the Canticles (Grail, Jerusalem Bible, RSV) while in the ICEL largely the NAB is used. Likewise for the Scripture Readings, the 3 volume may use any one of 5 translations (JB, RSV, Good News, Knox Bible, New English Version- generally the first two) while the ICEL uses the NAB
The 4 volume is a bit more user friendly I find. You do not have so much page flipping as in the UK version. The Ordinary is also shown quite nicely and clearly so one can see how the parts link together whereas with the UK version it is not apparent at all. The UK version has the slightly unsettling habit of putting some of its rubrics in funny places.