2020-06-23 : Because it's a loyalty ability, Chandra's last ability isn't a mana ability. It can be activated only any time you could cast a sorcery. It uses the stack and can be responded to.

2020-06-23 : If you don't play the exiled cards, they remain exiled.

2020-06-23 : If you have no cards in hand, you still exile three cards while resolving Chandra's first ability and you may play them this turn.

2020-06-23 : You must follow the normal timing permissions and restrictions for the exiled cards for Chandra's first and last abilities. If one is a land, you can't play it unless you have land plays available.

2020-06-23 : You'll still pay all costs for a spell cast this way, including additional costs. You may also pay alternative costs if any are available.