Xjyuk

Question about Actions vs Bonus Actions, vs Reactions:

In a game I was DMing recently (5e), the cleric had 2 ghouls on him. He took his reaction to dodge the attacks of the ghouls, and on his turn he took his action to try to use turn undead, then used his bonus action to dash from the ghouls. If he was able to do this, that was some pretty smart use of all of his actions, but I thought it seemed like using a reaction to keep dodging every attack that comes near you would make you really strong in combat.

Can you can even use disengage as a bonus action, or maybe even dodge?

From what I've read you could only use Dodge or Disengage as an action, unless you had some class feature that worked otherwise. He argues, though, that dodging is a bonus action, and thus he is able to do it.

Sorry that there's a lot to cover, but anything would really help me with this before my next game comes up.

The Dodge and Disengage actions are primarily considered as an option to take as your primary Action (PHB p.192):

Dodge

When you take the Dodge action, you focus entirely on avoiding attacks. Until the start of your next turn, any attack roll made against you has disadvantage if you can see the attacker, and you make Dexterity saving throws with advantage.

Disengage

If you take the Disengage action, your movement doesn't provoke opportunity attacks for the rest of the turn.

There are some classes however, that allow you to "Dodge", but these class features aren't the same as the Dodge action. For example, the Rogue's Uncanny Dodge feature halves damage, as a reaction (PHB p.96):

Starting at 5th level, when an attacker that you can see hits you with an attack, you can use your reaction to halve the attack's damage against you.

The Monk does gain a feature to allow them to Dodge as a bonus action, at the cost of a Ki Point (PHB p.78):

Patient Defense

You can spend 1 ki point to take the Dodge action as a bonus action on your turn

And then a feature similar to the Rogue's Uncanny Dodge (PHB p.78):

Starting at 3rd level, you can use your reaction to deflect or catch the missile when you are hit by a ranged weapon attack.

Outside of this, there is very little that allows a player to make these actions as a reaction, or a bonus action.

The only other suggestion I might make is that Matt Mercer's Critical Role series allowed players to use their Bonus Action as "another Action", to basically make things more cinematic. This was generally a spur of the moment decision to keep things moving, rather than to stop the game and check the rules, however.

Let's refer to this handy chart.

Dash (PHB 192), Disengage (PHB 192), Dodge (PHB 192), Ready (PHB 193), and Cast a Spell (PHB 192) are all Actions. You get to use one of those per turn during ordinary cases. Note that Turn Undead is an Action.

The only Reactions possible are an Opportunity Attack (PHB 195), a Readied Action (PHB 193, must have used Ready on their most recent turn), or a Cast (PHB 192) spell that has a casting time of a Reaction. You get at most one of these a round (so, it is refreshed only when their initiative comes up).

Bonus Actions can be an Offhand Attack (PHB 192,194-195), when using the Two-Weapon Fighting rule, a Cast (PHB 192) spell that has a casting time of one Bonus Action, or a Class Feature (PHB, see specific Class Features) with a time of Bonus Action. You get one Bonus Action per turn.

So, given this information, it appears that your intrepid Cleric took three Actions in a single turn. Short of any Class Feature or Racial Feature that might provide an explanation for this, this was definitely an abuse of the action economy.

To be sure though, check to see if they're using a particular Race, Feat, or Class Feature that might have allowed this action economy. I'm not aware of any such items that would match this, since action economy is tightly regulated to avoid the sort of balance problems you witnessed in that encounter.

The Cleric could probably still have won, but it would be a lot more challenging. Turn Undead on one, hopefully kill the other within 10 rounds, do some healing, if possible, then deal with the other. Or, use every single Action to Dash in the opposite direction to safety, whatever that might mean.

It'd be worth your time to reread the rules and refresh your memory, and maybe even find a handy chart like the one I've linked for moments like this. Also, whenever a player does something that seems genuinely Too Good To Be True, it never hurts to double-check the rules, just in case.