Nucleophiles (ie, nucleus loving) are the species that have atleast one lone pair of electrons for donation, ie, they can also be called Lewis bases. Among the given, $BH _{3}$ and $AlCl _{3}$, being electron deficient, behave like electrophiles while $\overset{ยจ}{N} H_{3}$, due to the presence of a lone pair over $N $, is a nucleophile