The Problem With Three Goblins
It doesn't take a genius to realize that, in adventure-based design, not every goblin is equal. If you take three goblins together, they are going to be, naturally, more challenging than taking on each goblin individually. This means that a default goblin might be a greater or lesser challenge depending on how many OTHER goblins it comes with.
That might be a problem, if you're looking to get some sort of budget for your adventure.
4e had a significant innovation in encounter design in the form of "group roles" for monsters: Minions, Standards, Elites, and Solos. Each one filled a different role in designing encounters — minions were more numerous, but weaker; elites were less numerous, and stronger.
You can apply this same logic to encounters in adventure-based design: some encounters will be "minions", that is, smaller, weaker encounters. Others might be "solos," or bigger, stronger, more robust encounters.
The Types of Monsters
Essentially, this means that some monsters are meant to be encountered in groups. These monsters are individually weak, but throw enough of them together, and they make a good encounter's worth of damage and resource-draining.
Other monsters might be meant to be encountered alone. They could be added to groups, but they don't need the groups to survive. They have an entire group in themselves.
Intentions Gone Awry
Now, just because your standard goblin might be a group monster doesn't mean they'll be encountered in a group. It means that the group tries to stay together, probably guarding each other's backs and taking turns on a watch, for instance, but it doesn't mandate that. A clever group might be able to lure out the goblins one by one and off them. This is good — good strategy should be rewarded. Similarly, your standard, say, beholder might not normally be encountered in a group — just one beholder is enough for a whole party. However, nothing says that the beholder can't join up with a troupe of drow for a while, if it wants, and be encountered that way — perhaps in a way that is effectively TWO encounters at once.
This all works fine. The quantity-guidelines are just that: guidelines. They're things to aim for. A group of goblins tries to fight as a unit. That doesn't always work, but they try to have that happen, if they can.