This is a case of "requirements" and "restrictions". He will be forced to block with all 3 creatures.
509.1c. The defending player checks each creature he or she controls to see whether it's affected by any requirements (effects that say a creature must block, or that it must block if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of blockers is illegal. Tapped creatures and creatures with unpaid costs to block are exempt from effects that would require them to block.
Example: A player controls one creature that "blocks if able" and another creature with no abilities. An effect states, "Creatures can't be blocked except by two or more creatures." Having only the first creature block violates the restriction. Having neither creature block fulfills the restriction but not the requirement. Having both creatures block the same attacking creature fulfills both the restriction and the requirement, so that's the only option.
Let me get one thing straight with you. When Condor and an official ruling disagree, 50% of the time the official ruling gets reversed later. The other 50% of the time, the rules get clarified/changed to make the ruling right when it really wasn't before.
If I attack the the Berserkers, and Hunt Down one of his creatures, what happens? Is my opponent forced to block with all three of his creatures?
509.1c. The defending player checks each creature he or she controls to see whether it's affected by any requirements (effects that say a creature must block, or that it must block if some condition is met). If the number of requirements that are being obeyed is fewer than the maximum possible number of requirements that could be obeyed without disobeying any restrictions, the declaration of blockers is illegal. Tapped creatures and creatures with unpaid costs to block are exempt from effects that would require them to block.
Example: A player controls one creature that "blocks if able" and another creature with no abilities. An effect states, "Creatures can't be blocked except by two or more creatures." Having only the first creature block violates the restriction. Having neither creature block fulfills the restriction but not the requirement. Having both creatures block the same attacking creature fulfills both the restriction and the requirement, so that's the only option.
MTG Rules Adviser/Advisor
Comp Rules, FAQs, and DCI Rules
But what happens if, instead, one of the three creatures my opponent has is tapped? Or has "~ can't block" Or has shadow?
The "must block" effect does nothing.
The Gorillas can't be blocked by only two creatures.
Butcher of Words.
Made by Spiderboy4
Gotcha. Alright, I'm all set here.
*goes off to design a weird deck using Summit Apes, Gorilla Berserkers, Gaea's Embrace, and Hunt Down*