Fish discrimination! HELP!

hi everybody. i recently purchased three 1-1.5" goldfish, and two of them keep following the third one and shoving their mouths into his sides (not gills) and butt. is there anything wrong with this? i put the third one into a 15 gallon container for now, but im a little worried as his fin tips have turned black.

i know this is the general freshwater section, but the coldwater section never responds to my questions.

This Is Urgent!

id rather not buy another fish tank, so i guess id keep him in the tub and put in a filter. i might realese him back in after the other fish forget about is weakness. goldfish aint that smart.