Pretty print a tree from minimal data input
Nice solution, my suggestions:

Short variable names: variables like camt and cname can be extended to currentAmount and currentName. It's better to have longer names to improve readability. For example, I am not sure what stem stands for.

Types and input validation: there are a lot of casting in the methods which are not validated. Accepting an array of Object without any validation is a recipe for troubles. Using a class you can solve both problems, something like this:
class Node{
    private int depth;
    private String itemName;
    private int amount;
    public Node(int depth, String itemName, int amount) {
        // Input validation here
        this.depth = depth;
        this.itemName = itemName;
        this.amount = amount;
    }
    // Getters, etc.
}

So this code:
Object[] o = tree.get(i);
int cdepth = (int) o[0];
String cname = (String) o[1];
int camt = (int) o[2];

Can become:
Node node = tree.get(i);
int currentDepth = node.getDepth();
String currentItemName = node.getItemName();
int currentAmount = node.getAmount();

You can also make the properties of the class final to have an immutable node.

Constants: the strings \--- and +--- are repeated multiple times. Consider creating constants for such special strings so that they will be easier to change in the future.

Functionality: If the list is not properly built, the tree will not be printed correctly. For example:
The input:
1 a 1
2 b 1
3 c 1
4 d 1

Produces the correct output:
\---a x 1
   \---b x 1
      \---c x 1
         \---d x 1

But if we add one more node in the middle:
1 a 1
2 b 1
3 c 1
2 e 1
4 d 1

The output becomes:
\---a x 1
  +---b x 1
  |   \---c x 1
  \---e x 1
      \---d x 1

Now d changed its depth from 4 to 3 and e became its father.
Providing a well-formed list of nodes with corresponding depth is not a simple task if the tree grows in size. A solution might be to create a Tree class that knows how to add and remove nodes. Then the treePrint method will do a Pre-order scan of the tree and print the nodes on the way.