Your the only one rimuru decided to bring alone in his recanation however your not blood related so no you hajima and rimuru are not triples however ciel made it so that you guys will be childhood friends and will go to the same high school and class also this AU takes place in the chapter where hajima and rimuru met the class and when Kouki Amanogawa challage hajima to a fight for kori and the other girl yue tio Shea Chloe and sakura