When I said of the hook for Seudo Shelishis, I meant off the hook for having to make Motzie & recite Birkas Hamozon thrice in 1 Shabbos, only on Yom Kippur & Erev Pesach.
On Shabbos Erev Pesach, one should eat a respectable meal after Mincho Gedolo preferably before last 1/4 of day, and have in mind Seudo Shelishis, despite consuming Shehakol such as fish & chicken etc; Of-course Seudo Shelsihis Zemiros can still be sung even without a formal haMotzie on Shabbos Erev Pesach.