Russian imperfective and perfective aspects, one time non repeated actions

Question

I have a problem understanding the use of imperfective in some sentences that appear to me as "one-time non repeated actions". Some of them use the imperfective form and some others use the perfective form. Is there a rule of which one to choose?

Мне нужно сходить в магазин
> Imperfective. In my head it sounds like you need to go there multiple times

Я уже сводил сына в бассейн
> Here, I don't understand why we don't use свести, since it's a finished action

Сбегай, пожалуйста, в супермаркет
> Here, it's perfective and I understand why.

Could you explain? Thank you :)