Questions on the ordering of triggered attacks.

[Gavin O.]

Let's say you have the feat Crossbow Expert, with a Rapier in one hand and a hand crossbow in the other. You're a Battle Master Fighter with the Trip Attack maneuver, and your target is flying. Can you take the attack action and attack with your Rapier (PHB 194 specifies that you can attack a "location" even if there's no enemy there.), trigger crossbow expert, and before your Rapier attack happens, shoot your target with a crossbow bolt, use the trip attack manuever, and have them fall into the reach of your melee attack? Would you then have advantage on that melee attack because the target is prone?

More generally, when exactly are you able to take bonus action attacks that trigger "when you take the attack action" or "When you use the attack action and attack"?

 

[Caliban]

Let's say you have the feat Crossbow Expert, with a Rapier in one hand and a hand crossbow in the other. You're a Battle Master Fighter with the Trip Attack maneuver, and your target is flying. Can you take the attack action and attack with your Rapier (PHB 194 specifies that you can attack a "location" even if there's no enemy there.), trigger crossbow expert, and before your Rapier attack happens, shoot your target with a crossbow bolt, use the trip attack manuever, and have them fall into the reach of your melee attack? Would you then have advantage on that melee attack because the target is prone?

More generally, when exactly are you able to take bonus action attacks that trigger "when you take the attack action" or "When you use the attack action and attack"?

Per Jeremy Crawford, with most bonus actions you get to decide the timing. He said this in reference to the Shield Master bonus action shove, but it would apply to this as well. If you do take the bonus action, you are then locked into using the "Attack Action" for your main action. If they are prone when you make your melee attack, you would have Advantage as normal.

https://www.sageadvice.eu/2015/01/29/shield-master-feat/

(side note: Weirdly, the monk bonus action "flurry of blows" specifies that it comes "immediately after you take the Attack Action" while the bonus unarmed strike that doesn't cost a ki point doesn't have this restriction.)