What causes isotopes?
Isotopes are atoms of the same element which have a different number of neutrons.
There are three isotopes of hydrogen. All have one proton - if they had two protons they'd be a helium atom!
The three isotopes are:
The difference in mass is the result of the differing numbers of neutrons in the isotopes.
H-1 has 1 proton and 0 neutrons = total mass is 1
H-2 has 1 proton and 1 neutron = total mass is 2
H-3 has 1 proton and 2 neutrons = total mass is 3
Protons and neutrons have a mass which is extremely close to 1 amu, it's so close that you can typically just round to 1. Electrons have virtually no mass - so I haven't discussed them in this answer.
Here's a video to help with this concept.
Hope this helps!