What three things brought Greeks together?

Answer 1

The Greek city-states spread much further afield - from Massalia (now Marseilles) throough Sicily and southern Italy, the Greek mainland, Asia Minor, North Africa and Black Sea.

1. They shared religion, language and culture, and attributed their origin to a founding ancestor, Hellen which is why the called themselves (and still do) Hellenes, and the Hellenic world (Greece/Greek is a name the Romans saddled them with when they confused them with a southern Italian tribe the Graeci).

2. The cities outside the mainland were generally colonies of mainland cities, and usually honoured the bond with their mother city.

3. They regarded other peoples as barbarians, and so the feeling of uniqueness and opposing external enemies provided a unifying force.

That said, the cities were not 'joined' or unified. The Amphictyonic Council was the arbiter of Hellenic cultural matters, however the cities were independent states, and were more often at war with each other than barbarians. From time to time some cities formed defensive alliances against each other or external enemies.

Answer 2

The different Greek city states had the same language and religion. They played sports against each other in the Olympic games.