Agios Stephanos, Agios Sostis (sometimes called Nisaki) and Pelagonisos are three small islands situated northwest of the harbor of Mesta (southwest Chios, so on the western side of the island).
All three islands are only accessible by private boat. Agios Stefanos has a beach. This island takes it's name from the church that is built just above the beach. This beach is a pebble beach and it is completely deserted, ideal for naturists. I understand there are also remains of Hellenistic watch-towers, but I haven't been able to localise those on satelite pictures.
Nisaki and Pelagonisos have a rocky coastline and you cannot find any beaches on them.