We call a particular configuration of 3 particular kinds of quark "a proton", and another one "a neutron" (there are also anti-protons). In general, combinations of an odd number of quarks (3, 5, 7, ...) are called baryons (only combinations of 3 and 5 have been proven to exist, any more are only speculation). There are also some unstable but observable combinations of equal numbers of quarks, which are collectively called mesons (combinations of 2 and 4 quarks have been proven to exist, others are only speculated).
Now, why are the combinations of 3 quarks the only ones (that we know of, at least) that are stable is a much more complicated question related to properties of the strong force.
I'm not smart enough to answer this, but it seems to me that the question is along the lines of, "Why do all verbs describe action?" Because if they were different, they'd be something else.
We call a particular configuration of 3 particular kinds of quark "a proton", and another one "a neutron" (there are also anti-protons). In general, combinations of an odd number of quarks (3, 5, 7, ...) are called baryons (only combinations of 3 and 5 have been proven to exist, any more are only speculation). There are also some unstable but observable combinations of equal numbers of quarks, which are collectively called mesons (combinations of 2 and 4 quarks have been proven to exist, others are only speculated).
Now, why are the combinations of 3 quarks the only ones (that we know of, at least) that are stable is a much more complicated question related to properties of the strong force.
Equilibrium all the way down.
I'm not smart enough to answer this, but it seems to me that the question is along the lines of, "Why do all verbs describe action?" Because if they were different, they'd be something else.
This could be asked at a few different layers of abstraction but I'd start here https://en.wikipedia.org/wiki/Color_confinement
Higher numbers of quarks can get bound up together but aren't stable.
https://en.wikipedia.org/wiki/Tetraquark
https://en.wikipedia.org/wiki/Pentaquark