Three or more people who eat a meal together (with bread) are obligated to say Zimmun (lit. "invitation", where they recite the Birkath HaMazon together). If they forgot and each one recited the Birkath HaMazon (Grace after Meals) on their own, they have lost the opportunity to say Zimmun and cannot say it together after the fact.
If only one of the three forgot and recited the Birkath Hammazon by himself, or even if he did so deliberately, the remaining two can include him in the Zimmun and fulfill their obligation. He, however, does not fulfill his obligation with this Zimmun even though he was included in it, because one cannot fulfill one's obligation of Zimmun Bedi'abad (after the fact). This also assumes that they are all adults with no boys under the age of 13.
The Rema adds that in a case where one of the three combined with two others for the Zimmun, the remaining two are unable to do a Zimmun. This is because he has already fulfilled his obligation
(See Shulhan 'Aruch 194:1 with Rama. Mishnah Berurah ibid 4. Kaf Hahayim Oth 2, 4 & 5)
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