もしお時間がありましたら、ご意見を頂けると嬉しいですって英語でなんて言うの？

どれも丁寧な言い方ですが、３つ目が最も丁寧です。

I'd appreciate it of you could ...は「…していただけるとうれしいです」という意味の丁寧な依頼の表現です。

＜ボキャブラリー＞
if you have time = 時間があれば
appreciate = 感謝する
opinion = 意見
grateful = 感謝している

「①　If you have some time, it'd be great to hear your thoughts」は日本語にすると「少しお時間がありましたら、ご意見を聞ければ有り難く思います」。

ジュリアン

丁寧のレベルを少し下げました。

とても丁寧な文章は既に回答されていますので、
失礼ではなく、かつ、そこまでかしこまっていないバージョンにしました。

普通の文章ですが、失礼な感じは受けないと思います。
堅苦しくない言い方にしてみました。

「お時間のあるときに、ご意見をいただきたいと思います。」
こんな感じです。

If you would like to send an email to someone asking them for their feedback/opinion on something, you can say something like "I was just hoping that you can give me your feedback/opinion on something if you have the time." or "Could you please give me your opinion/feedback on this if you have the time?".

Email is a great way of communication these days. It is used by everyone and moreseo in business situations. Now we have quicker ways of communication like with Smartphones so the message can be received earlier.

This sentence is a very polite way of addressing your boss if you must ask him something by email. He will probably respond ñveru quickly unless he is very busy.

Here are three ways that we can phrase this question and get feedback from our boss. In the first two examples we can either use the noun, "time," or, "a moment," interchangeably for the same meaning.
In the third example, we can see that the phrase, "when you have a moment," can either be put at the the beginning or end of the sentence.