The atomic number of iron (fe) is 26. Electronic configuration of Fe with 26 electrons is

1S2

2S2

2P6

3S2

3P6

4S2

3d6

Fe3+ means 926-3) electrons = 23 electrons.

Due to the fact partially filled orbital configuration is stable enough,

We can say

Fe3+ = 1s2  2s2 2p6 3s2 3p6 4do 3d5

The outer s and d orbitals dictates its configuration that orbitals therefore have 5 electrons in its outermost part. Since the outermost shell can be occupied by a maximum number of 10 electrons it will have 5 unpaired electrons.

Also

Fe-1s2 2s2 2p6 3s2 3p6 3d6 4s2

When Fe+ loses one electron = 1s2 2s2 2p6 3s2 3p6 3d6, 4s1

And when Fe loses 2 electrons, Fe2+ =1s2 2s2 2p6 3s2 3p6 3d6 4s0

When Fe loses 3 electrons its, Fe3+ =1s2 2s2 2p6 3s2 3p6 3d5

For Fe to convert to Fe3+ it will have to lose 2 electrons from 4s orbital and 1 electron from 3d orbital. The resulting electronic configuration becomes 4s^0 and 3d^5.

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