hmmm, do you neglect your son n tend to tend to your daughter more?
at this age of 3, they tend to want more attention from parents, thus the jealousy.
probably allow them to do more bonding together?
my cousin's elder daughter also like that to her younger sis.
n that time she was 5 n sis was 3.
but now, they r like best friends.
i think your son is still very young to understand this .
engage him more in activities with your girl, like, 3 of you play together.
let him help you change, n feed her.
let them watch edu progs together.
you need to assure him often that mummy n daddy stil loves him alot.
maybe can spend one on one time with him n let ya dear husband take care of your girl awhile?
kids r like that, need to feel secure.