1.I am writing to apply for a job in your music shop.
这个时态没错,表示正在写,后面 apply for sth. 申请/应聘
2.I have always had a particular interest in music.
I have had/got 我有
3.I have attached my resume to support what was written above.
what was written 表示您附上的简历已经是写好了的用过去时.