Comment by tsimionescu

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.