for l in {0..999}
do
  id=$(printf %03d $l)
  alias server$id="ssh user@server$id"
done

bash-4.2$ alias | grep server | head
alias server000='ssh user@server000'
alias server001='ssh user@server001'
alias server002='ssh user@server002'
alias server003='ssh user@server003'
alias server004='ssh user@server004'
alias server005='ssh user@server005'
alias server006='ssh user@server006'
alias server007='ssh user@server007'
alias server008='ssh user@server008'
alias server009='ssh user@server009'