How Alexander, Chandragupta Maurya and Seleucus were related to each other?
According to Wikipedia page Alexander invaded India in 326 BC in the battle of Hydaspes. Later he returned back.
After the death of Alexander, Seleucus, who was commander in Alexander's army attacked India but he was defeated by the king Chandragupta Maurya So Seleucus and Chandragupta Maurya signed a treaty as well and Seleucus appointed an ambassador who was no one but the Megasthenes.
After the Seleucid-Mauryan war Chandragupta Maurya married to Durdhara who was daughter of Seleucus. So, Chandragupta and Seleucus were relatives like this, and as Seleucus was commander of Alexander's army so Chandragupta was indirectly related to him also.