Advise on my cats behaviour?

hi, i wanted some advise, i have recently taken in a 12 week old female kitten, due to a friend not being able to keep her, i already have 2 females 12 and 14 months old. at first i thought the older of te two would be the problematic cat but she seemed to follow the usual signs, hissing and snarling and then eventually interacting with the new kitten, my 12 month old who is always laid back is in fact my main concern, she is completely withdrawn hides anywhere she can to get out the way and eats very little, her normal cuddle traits are completely gone, i am really worried any advise or reassurance would be grately appreciated.