I know there are other tutorials out there telling about this but I have yet to see one on this site, so here is my version of it (what can I say I am in a tutorial mood).

I'm not gonna tell you how to make a chest, someone else can do that. You're going to want a variable named something like chest. In the chest event set a change variable chest to random 1*3, then make a fork option when variable chest is set to 1 equal/ same with no else case. It should look like this

<>Variable Ch:[0004Chest]Set, Randm[1*3]
Fork Optn:Varbl:[0004Chest]-1
<>
END CASE


Now copy and past the fork option until you have three of them but change the conditions so variable chest is set to 2 on the second and to 3 on the third. Like this

<>Variable Ch:[0004Chest]Set, Randm[1*3]
Fork Optn:Varbl:[0004Chest]-1
<>
END CASE
Fork Optn:Varbl:[0004Chest]-2
<>
END CASE
Fork Optn:Varbl:[0004Chest]-3
<>
END CASE


Ok now after you get that you will need to put an add item under the first fork option. I make it to add a potion. Then you should put some kind of message saying what the hero got, like "You got a Potion!"
Now do that under each one but put a different item. There you go an easy randomized chest.

In this example I just made it to were there are 3 possible items but you can have more.