When doing this, the Chometz must be consumed before the 5th hour of the day. In order to accomplish this, the minhag is to daven early and then split the meal into 2 (one for Shabbos day meal and one for seudah shlishis) and be done with both meals before the end of the 5th hour. (See Mishna Berura Siman 444 S”K 8)
There must be a short break between the 2 meals, as to ensure there is no ‘Bracha She’aina Tzericha’ (See Mishna Berura ibid.) The Chazon Ish (Igros Chazon Ish Vol. 1 188) maintained that this break must be for half an hour. Other Poskim say it can be for 15 minutes, and some even say it can be much shorter. It is best to try and wait a full half hour if the time permits it. If possible, go outside and take a short walk to ensure a proper separation between the 2 meals.
Usually it is proper to eat Seudah Shlishis on Shabbos after Mincha. Being that this year Shabbos is Erev Pesach and we can’t eat Chometz after the 5th hour, some Poskim allow it to be eaten earlier (See Mishna Berura ibid.)
However, some Poskim maintain that it should still be eaten after Mincha, and indeed eat a meal of fish or meat, or even fruit, but without bread, as is permitted by Seudah Shlishis, especially in extenuating circumstances. When eating Seudah shlishis later in the day, after Mincha, care should be taken not to eat too much, so that you have an appetite for the Seder. (See Mishna Berura ibid.) For Halacha L’ma’aseh, of course, each individual should consult their Rav.
