Answer
Ptolemy III was removed from succession because his mother, Berenice II, was disgraced by her husband, Ptolemy IV. Berenice II was Ptolemy IV's sister, and the two of them had a son together, Ptolemy V. However, Ptolemy IV was not a good ruler, and he was eventually assassinated by his own bodyguards. Ptolemy V succeeded him, but he was only a child, so his mother, Berenice II, ruled as regent.