Iberia Airlines Iberia Plus - IB check-in at LHR T3

Will be flying IB metal to MAD from LHR T3 in the next few days on a BA flight number. As a oneworld Sapphire, can I drop my bag more than 3 hours in advance?

Separately, would the agent be kind enough to let me stand-by (for free) for an earlier flight to MAD should space permit?

Yes you can as you have status.

European airlines do not do standby in the same way that US ones do. If you have a fully flexible ticket then you can cancel and rebook on the earlier flight (for a fee). Otherwise sit down with a good book and wait!

By "next few days" you mean prior to March 25, right? That's the day IB begins operations from T5 at LHR.