suus, sua, suum declines just like tuus, tua, tuum.. it's a regular 2-1-2 adjective. the nominative plurals are sui, suae, sua. eius is not the plural of suus. eius is in fact the genitive singular of is, ea, id... the third person pronoun (he, she, it). eius is not from idem, idem is an adjective meaning "the same" and is composed of is, ea, id + dem... so the genitive singular is eiusdem.