Uiua, 7 bytes (SBCS)

<+/∘⊏⍖. 

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Explanation:

## (Top of stack on the left) ## Stack: [1 3 2] . # Copy input ## Stack: [1 3 2] [1 3 2] ⍖ # Index array by descending value ## Stack: [1 2 0] [1 3 2] ⊏ # Order the input ## Stack: [3 2 1] /∘ # Put the elements on the stack ## Stack: 1 2 3 + # Add the top two items ## Stack: 3 3 < # Test whether the second item is less than the first ## Stack: 0 

In the example, I've had to "spell out" the function each time rather than use a binding, because it would require 5 more bytes to annotate the signature:

Triangle ← (|1 <+/∘⊏⍖.) 

Uiua, 7 bytes 9 bytes (SBCS)

|1<+/∘⊏⍖. 

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Explanation:

## (Top of stack on the left) ## Stack: [1 3 2]   . # Copy input ## Stack: [1 3 2] [1 3 2]   ⍖ # Index array by descending value ## Stack: [1 2 0] [1 3 2]   ⊏ # Order the input ## Stack: [3 2 1]   /∘ # Put the elements on the stack ## Stack: 1 2 3   + # Add the top two items ## Stack: 3 3  < # Test whether the second item is less than the first ## Stack: 0 |1 # Function signature 

Edit: Added function signature (2 extra bytes) to conform to rules. Thanks to @Bubbler.