Linux shell script to reboot a few linux machines

Dear experts,

I am task with something I have never done before.

I am sure is easy for some of you :)

I need to execute a linux shell script from our network that will connect ssh to this devices and reboot them with 15 minutes difference.

I imagine it will look something like this:

 #!/bin/bash

USERNAME=someUser
HOSTS="host1 host2 host3"
SCRIPT="pwd; ls"
for HOSTNAME in ${HOSTS} ; do
    ssh -l ${USERNAME} ${HOSTNAME} "${SCRIPT}"
done
Need to connect the reboot command with the ssh connection

for p in server1 server2 server3
do
  ssh ${p} sudo reboot
  sleep 900

 I need it to connect and reboot after 15 minutes from the first machine rebooted.

Thanks