The rules for cover state:
If a target is behind multiple sources of cover, only the most protective degree of cover applies
In most situations, determining which cover is most protective is fairly straightforward. Against almost all attacks, the order of protectiveness is half < three-quarters < total. What I want to know is whether that is the only way to interpret the 'most protective cover' statement. To give a concrete example of a situation where this might be relevant, let us look at fireball.
In this question on fireball ignoring total cover, it is established that fireball can ignore total cover by going around corners. However, the spell description does not explicitly say that the target does not gain a bonus from cover for the saving throw, as sacred flame does. This means a target with three-quarters cover but not total cover could possibly get the +5 bonus to their Dexterity saving throw.
If a target has total cover and three-quarters cover, and is exposed to fireball going around a corner, the +5 to the Dexterity saving throw provided by three-quarters cover would be more protective than the no benefit provided by total cover.
This is just an example, so let's not get too caught up in the details of it (it may not even work like I have put it, but that is not the thrust of the question).
What I really want to know is:
If a target has total cover and three-quarters cover, and a situation arises where the benefits of three-quarters cover would be more beneficial than the benefits of total cover, does the target get the benefits of three-quarters cover or total cover?