Goblinoid isn't a race; it's a family of races.
Goblins, hobgoblins and bugbears are distinct humanoid races, each with their own physical characteristics and culture.
The word goblinoid in their stat blocks is a tag:
A monster might have one or more tags appended to its type, in parentheses. For example, an orc has the humanoid (orc) type. The parenthetical tags provide additional categorization for certain creatures. The tags have no rules of their own, but something in the game, such as a magic item, might refer to them. For instance, a spear that is especially effective at fighting demons would work against any monster that has the demon tag.
Note that tags have no inherent meaning. While the goblinoid tag applies to creatures that are closely related to each other, other tags like shapechanger apply to a disparate collection of creatures, such as vampires, lycanthropes and doppelgangers.
The Monster Manual has separate sections for goblins, hobgoblins and bugbears. This is the first clue that they're not just one species. In their respective sections, we find these quotes:
Goblins belong to a family of creatures called Goblinoids. Their larger cousins, hobgoblins and bugbears, like to bully goblins into submission.
Hobgoblins belong to a family of creatures called Goblinoids. They are often found lording over their cousins, the smaller goblins and the ferocious bugbears.
Bugbears are often found in the company of their cousins, hobgoblins and goblins.
Goblinoids are consistently referred to as a family of creatures and the different races are referred to as cousins of each other.
Volo's Guide to Monsters contains playable monster races; goblins, hobgoblins and bugbears are presented as distinct options. The book also goes into more detail about these races as a group and individually:
In bygone times the goblinoids were distinct from one another, with separate faiths and different customs ... What the goblins, the bugbears, and the hobgoblins were before their gods bowed to Maglubiyet no longer matters. Now they are, first of all, followers of Maglubiyet ... Each race his its own tendencies, outlook, culture, and gods. But Maglubiyet's hand joins them together, just as he made all their gods parts within a greater whole.
The common thread between goblinoids is their shared worship of Maglubiyet.