Fanaelialae
Legend
From a very limited perspective, sure.3 intelligence is dumber than a baboon.
However, baboons don't get a language. A character with 3 Intelligence does. Clearly there's more going on here than a single number can express.
Let's look at the default lifting and carrying rules on page 176 of the 5.0 PHB. A human with 20 Strength can lift 600 lbs. An ogre with 10 Strength can lift the same amount. Does that mean that the human and the ogre are equally strong? Clearly not! The human gets a +5 on all Strength-based checks, while the ogre gets +0. But if we were to look at that 600 lb lift in exclusion to all other factors, one could come to the wrong conclusion that there two creatures are equally strong. I would say that your baboon example is a similar case.
Just because a human and a baboon have the same Intelligence, does not mean that the human is only capable of intellectual feats that a baboon is capable of. Page 317 of the 5.0 MM lists an Ape's Intelligence as 6. That's quite possible with rolled stats. Are we to now assume that any PC or NPC with a 6 Intelligence is limited to the intellectual capacity of an ape, or are we willing to allow that a single value maybe doesn't tell the whole story?