$hosts ="host1 host2 host3"

# use ssh-keyscan to populate your "known_hosts" file first:
ssh-keyscan $hosts >>~/.ssh/known_hosts

# then append your key on the remote hosts:
for h in $hosts
do
  echo "*** copying key to $h ***"
  cat ~/.ssh/id_rsa.pub | ssh $h 'cat >>~/.authorized_hosts'
done