The rules for the ballista on p.255 of the DMG state that to make an attack the weapon must be loaded, aimed and fired. Each of these takes one action. However, could each of these actions be performed by a crew of three, with one character to perform each step, thus allowing the ballista to be fired once per round?
My assumption is yes, but there's nothing textual to support this. Am I missing something?
It takes 3 actions, as you cited, so all your party would need to do is perform the actions on their turns in the proper order of:
This is strictly a RAW answer which respects the action economy of 5e. The ballista does not state that each action requires 1 round, simply that each function requires 1 action. Working as a team to load, aim and fire such a simple weapon is hardly unexpected of seasoned adventurers.
The fact that multiple people are trying to interact with the same object means time, as seen by the object, cannot be smooshed into a shorter time simply because people are using the fuzzy lens of 5e combat.
You can't reverse the lens to create fuzzy pixels. The lens used for RPGs is one way. It is how we convert reality into game mechanics. Turning that lens around and using gaming mechanics to alter reality is a misuse of that.
My first assumption would be; "no". Even with "house-rules" the person doing the aiming should at least be the same as the one firing the weapon.
Would you stop aiming and permit somebody else to step in to fire the weapon? Meaning, even if initiative order was intact and in sync., there would at best be a two-round delay.
Even to get it to two rounds would take three people using actions because of initiative order:
