why length 3 key in utf8 use 11 bytes?

[freebat]

create index ub_city_key on ub (city(3));

           id: 1
  select_type: SIMPLE
        table: ub
         type: ref
possible_keys: ub_id_key,ub_city_key
          key: ub_city_key
      key_len: 11
          ref: const
         rows: 4340
        Extra: Using where
1 row in set (0.00 sec)


Why the key_len is 11 not 9?

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