How many neutrons will an atom have it has an atomic number of 26, an atomic mass of 58 and a charge of +3?

Recall that both 1 proton and 1 neutron each have a mass of 1 amu. (Electrons have a mass on the order of 10⁻⁴ amu so we can effectively ignore it.)

Now,

atomic number = number of protons = 26

hence the protons have a total mass of 26 amu. The atomic mass is the sum of the masses of protons and neutrons, so

atomic mass = mass of protons + mass of neutrons

mass of neutrons = 58 - 26

mass of neutrons = 26

and again, each neutron has a mass of 1 amu. So there are 26 neutrons in the nucleus of this atom.

Don't let the extra positive charge fool you; both the atomic number and mass are fixed, so the discrepancy must be due to the absence of 3 electrons.