Answer: 2 protons
Explanation: Isotopes are atoms of the same element with the same atomic number but different atomic mass. The difference in the atomic mass is as a result of difference in the number of neutrons. Atomic number of an element is the number of protons in the nucleus of an atom while atomic mass of an element is the sum of the number of protons and neutrons in the nucleus of an atom. Atomic mass = number of protons + number of neutrons. Since helium-3 has 1 neutron and an atomic mass of 3, there its number of protons = 3-1 = 2.