To find the atomic number of the atom, we can use the given information about the number of protons and neutrons. Let's represent the number of protons as "p" and the number of neutrons as "n".

Given that the mass number (A) of the atom is 45 and there are three more neutrons than protons, we can write the following equations:

A = p + n

n = p + 3

Substituting the second equation into the first equation, we get:

A = p + (p + 3)

45 = 2p + 3

Solving for "p," we get:

2p = 42

p = 21

Therefore, the atomic number of the atom is 21, which corresponds to the element Scandium (Sc).