Is the dragon fouling the horse-monster's attack?

A is a Large (tall) creature armed with a longspear, giving the creature a reach of 20 ft. B, a Small creature that's hovering 5 ft. above the ground, has 10 ft. between him and the the Large (tall) creature. C, a Medium creature that's standing on the ground, has 15 ft. between him and the Large (tall) creature, in line with the Small creature. Imagine this in three dimensions:

Does flying-5-ft.-up Small B grant standing Medium C soft cover from Large (tall) A's longspear melee attacks?

Note: In context, A was an equiceph from Miniatures Handbook 61 (I like obscure monsters—don't judge), B was a PC who had assumed the form of a Small hex dragon from Dragon #343 39, and C was the fast-moving scout. There was a lot of talk at the table, but I think at least part of our (okay, my) confusion stemmed from drawing lines from corners of squares versus drawing lines from corners of cubes. (I know, right?) Even with miniatures and flight cubes (1" numbered blocks used to represent altitude), we were undecided on whether the dragon granted the scout soft cover from the equiceph's longspear attacks. Eventually, we compromised, assessed a −2 penalty on the equiceph's attack roll, and moved on, but I'm curious as to the correct way to assess this situation.

No, B doesn't grant C cover.

This is because it is probably irrelevant if you look at the battle from above or from the side. If you can draw unblocked lines in any projection, there is no cover.

The same picture from any side will be:

According to "Big Creatures and Cover":

Such a creature can choose any square that it occupies to determine if an opponent has cover against its melee attacks.

So this answer is probably true1, even if we think that drawing lines from equiceph's feet would be an overstretch. And nothing technically stops you from doing the last.

1: There is one somewhat murky point.

Cover rules also say:

When making a melee attack against a target that isn’t adjacent to you (such as with a reach weapon), use the rules for determining cover from ranged attacks.

It is hard to say which of the quoted parts overrides another when you make a melee attack but not against an adjacent target. I personaly lean towards the interpretation that you may use corners of any square (or cube) contained in the space you occupy, and than use rules for ranged cover in regards to this chosen square (or cube).

What confuses me is that Cover initially speaks about squares not spaces, but then brings up specific rule for squares being separable in a creature's space in case of melee cover only.

• Cover says, "To determine whether your target has cover from your ranged attack, choose a corner of your square." So for big creatures, doesn't mean choose any corner of the space you occupy but choose any corner of any cube you occupy? I'm good with that, but is there text backing up that reading? Oct 30, 2017 at 16:11
• @HeyICanChan Well, now I got what confuses you. I don't have such a rule at hand. There is little support for 3D in the rules sadly. My answer is based mainly on the fact, that despite different lenths and widths, creatures occupy squeres, so the same probably should go for heights (therefore cubes). It implies the same reach overhead to me, for example, so threatened areas should look the same from above, from sides and from the front/rear. And that in its turn implies that if you are concerned about 3D, you should consider 3D piramids from imaginary corner-to-corner lines, not just sectors. Oct 30, 2017 at 18:01
• Cool. Any suggestions on how I can update the question to make it clearer? Anyhow, it would go a long way toward making this answer awesome if it cited that drawing a line from any square (or cube — whatever) that the creature occupies is legit. (I don't remember anyone at the table taking out the laser line pointer and measuring from the equiceph's middle square. That's mindblowing.) Oct 30, 2017 at 18:12
• @HeyICanChan I've editted in info about squeres vs spaces. As for your question, I think it is fine as is. It was me who did not pay enough attention when I first read and answered it. Oct 31, 2017 at 9:16
• Seriously, I anxiously look forward to the downvoter's alternative answer. This seems pretty legit to me. (Y'know, except that the footnote doesn't go anywhere. ;-)) Nov 6, 2017 at 10:45