Trying to sort in alphabetical order of last name but needs to be formatted ID, first name, last name

No matter what I try with cut, awk, sed, or grep, it orders by ID first only then last name then first name when it should go in order of last name so it instead should look like

  f132b02: Kiara Acevedo
  f132a01: Caleb Barn

but instead it orders it by ID and not alphabetical last name and output looks like this

f132a01: Barn, Caleb
f132b02: Acevedo, Kiara

the information from the folder I’m retrieving the information is extracted from “/etc/passwd” and is trying to order the IDs below

f132a01:Barn  Caleb
f132a02:Casey  Nathan
f132a03:Sanchez  Ana
f132a04:Thomas  Jeff
f132a05:Cavhill  Jared
f132b01:Johnson  Andrew
f132b02:Acevedo  Kiara
f132b03:Felaccio  Steven
f132b05:Blotner  Sam
f132b06:Pereira  Brian

The code I’ve been trying is down below:

username, first name, and last name
sed -E 's/(.*), (.*) (.*)/1 2 3/' | 
sort -k3

echo

count=$(cut -d: -f1,5 /etc/passwd | grep f132 | wc -l)
echo "Number of students in the class: $count"