If I play Yare on Wall of Souls to block three attacking creatures that are 3/3, does the wall inflict 9 points of retaliatory damage to my opponent? Even though the damage from two of the three attacking creatures is enough to destroy the wall, damage from the third attacking creature must still be assigned to the declared blocker (i.e. the wall) correct?
A blocked attacking creature that is being blocked by a single creature must assign all its combat damage to that blocker. Furthermore, all combat damage is assigned at the same time by all attacking and blocking creatures, then it gets dealt all at the same time as it was assigned. Only afterwards are creatures checked for lethal damage, so all four creatures already dealt their combat damage at that time. And only after that do triggers go on the stack. So the Wall will deal the full 9 damage to the targeted player or planeswalker, even though it is already dead. And it is a single trigger because there is only one instance of combat damage being dealt, and the trigger condition doesn't look for creatures dealing combat damage, just for combat damage being dealt.
Combat damage is dealt at the same time (C.R. 510.2), so even if two or more creatures deal combat damage to Wall of Souls at the same time, its triggered ability will trigger only once (C.R. 603.2, especially C.R. 603.2c; C.R. 700.1), but even so, the combat damage is added up for the purposes of that ability. However, if creatures deal combat damage to Wall of Souls at separate moments (e.g., if some of them have first strike and/or double strike and others have neither [C.R. 702.7b, 702.4b]), then that ability will trigger once for each such moment (C.R. 603.2, 603.2c). Note that due to a recent Oracle text update, Wall of Souls's triggered ability can now target any "opponent or planeswalker" (C.R. 108.1). See also this thread and this thread.
By the same token then, if I played Yare on Will-o'-the-Wisp to block three attacking creatures, I would only have to regenerate Will-o'-the-Wisp once as the combat damage is dealt all at the same time, is that correct?
Yes, one regeneration shield is enough. It's not so much the damage that matters but state based actions that destroy lethally damaged creatures, and they do so only after the whole of combat damage has been dealt in a combat damage step.
Even if first strike or double strike is involved, and thus there are two combat damage steps, when the first shield is used up in the first combat damage step, the Wisp is removed from combat while the attackers remain blocked and usually cannot deal combat damage to the player/planeswalker they are attacking despite nothing blocking them anymore.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)
Even if first strike or double strike is involved, and thus there are two combat damage steps, when the first shield is used up in the first combat damage step, the Wisp is removed from combat while the attackers remain blocked and usually cannot deal combat damage to the player/planeswalker they are attacking despite nothing blocking them anymore.
Former Rules Advisor
"Everything's better with pirates." - Lodge
(The Gamers: Dorkness Rising)
"Any sufficiently analyzed magic is indistinguishable from science."
(Girl Genius - Fairy Tale Theater Break - Cinderella, end of volume 8)