Look up the electron configuration. It is
1s2 2s2 2p6 3s2 3p6 3d10 4s2 2p3, The 3+ ion is made by removing the last 3 electrons (the 4p3). So just count the electrons left. Another way to do this, and it may be easier is to look up As on the periodic chart. Mine shows the atomic number as 33 so I know the neutral element has 33 electrons. If I make it 3+ by removing 3 electrons then what's left must be 33-3 = 30 in the 3+ ion.
Number of electron in As3+ ion?
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