Hai teman! If that, the pronoun "mereka" is never used (only for humans), and only "ini" and "itu" are allowed as singular and plural, "it", "they", "its", "their", "them", for objects. And "ia" is allowed for "he", "she", "it" (only as subject, and not possessive), while "dia" is allowed as "he", "she", "his", "her", "him".

Selamat belajar! :)