You read fast, Eru!
When first reading the trilogy I too was peeved when Arthur and co. entered the picture... actually, I was peeved even before that, rolling my eyes at the blatant Tolkien "echoes" or what I thought were more like "ripoffs" when I first ventured through the work... :P:P
...but somehow, inexplicably, by the end of it all, after crying and laughing and having my heart ache because of Kay's words... I just went "wow". Somehow it all comes together very beautifully and makes sense... and the way that Kay intertwined his source materials, be they Tolkien or Celtic Legend, instead of being annoying became beautiful and admirably skillful in retrospect.
I dont even remember what is contained in the last book per se, my last re-read was a while ago and I remember the story as a whole...
Im glad you are enjoying it overall though, at least enough to keep going !