(3) I am writing as requested with my comments on your concerns regarding your investments.
は、普通、次のように解釈するものだと思います。
as requested ≒ as I was requested to do so は挿入句であって、
writing as requested with my comments
≒ writing (as requested) this letter with my comments
（ご要望どおり）私のコメントを添えた手紙を書いております
→ご要望どおり、私のコメントをお送りします
