I am talking about events taking place in the known future:
Would it be okay if I'll confirm around 3 pm?
or should it be
Would it be okay if I'd confirm around 3 pm?
What is the difference and which one is correct? Why?
Both Al and Mike are correct; you should use one of these forms:
Use (a) if you're in a situation where you're seeking the person's permission: that is, you need permission to either be able to confirm later, or permission to confirm later at the time you mention. With the (a) form, you're seeking the other person's input or approval, and their decision might completely alter your plans.
Use (b) if it's already established that the plans are agreed, and now you're working out logistics. You're already agreed to do whatever it is, and you've agreed that you can cofirm later, but now you're seeking to find out what the best time is for confirmation.
This is a subtle, idiomatic distinction more than one of syntax.
From a syntactic point of view, I believe, 'would' is a modal auxiliary verb, indicating possibility not certainty; whereas the auxiliary verb 'will' conveys definite intent of action.
I think both are incorrect. This is better, I think:
Say the first sentence to yourself without the I'll
I think it's obvious that that is incorrect.
Similar issue with the second example.
Sounds better, but still not very good.
I think a better way to go is