Hey there! So in order to answer this question we have to look at the electron configurations of those transition metals. Notice that those particular metals, Cs through Zn, are located on the 3d sublevel. D holds 5 orbitals, so thats 10 electrons in total that it can hold.
Now, we know that in order to have 3 unpaired electrons, we need to have 3d^7 configuration. Corrects? Ok, with that in mind, we are dealing with an element that has a +3 charge, meaning it has lost 3 electrons. Because our configuration is 4s^2 3d^10 for that row of transition metals, we can see that when we take aways electrons we first need to take it from a higher numbered orbital first, which is 4s^2. So that means we are only taking away 1 electron from 3d sublevel. In order to have 3d^7, we must have started with an element that had 4s^2 3d^8 electron configuration.
If you take a look at the periodic table, this configuration corresponds to Ni, or in our case Ni^+3.
Hope this makes sense, let us know if you need further clarification.