3:
/tellraw @a ["",{"text":"\u30b9\u30bf\u30fc\u30c8\u307e\u3067 "},{"text":"3","color":"yellow"}]

2:
/tellraw @a ["",{"text":"\u30b9\u30bf\u30fc\u30c8\u307e\u3067 "},{"text":"2","color":"red"}]

1:
/tellraw @a ["",{"text":"\u30b9\u30bf\u30fc\u30c8\u307e\u3067 "},{"text":"1","color":"dark_red"}]

スタート:
/tellraw @a {"text":"\u30b9\u30bf\u30fc\u30c8\uff01","color":"green"}