Does Harbinger's ability combo with other effects that allow you to cast minions, e.g. Elder Ruins, Troubled Town, Peculiar Port, etc.?

Yes! It's effectively a passive ability that can be used whenever you cast a spell on your turn. The golden rule, baby!

When I determine my three random squares, what if I roll doubles or triples?

Reroll any duplicated results until you have three unique squares.

When do I determine the random squares?

As soon as your Avatar enters the realm, during step 2 of setup.

Harbinger

Printings