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By specifying within the formula that the sum equals 100, you are essentially turning this formula field into a binary expression. When the expression is true (X + Y + Z = 100), then the formula returns a value of 1. When the expression is false (X + Y + Z ≠ 100), the formula returns a value of 0. You can format this field to use white text so it is will not be visible to the signer.
Note: If you want to display the added total to the signer as they enter new values, use a different formula field that has just [X] + [Y] + [Z]. This display formula should not be used in the following logic.
Next, you must turn the formula field into a trigger and create conditions that make it impossible for the signer to complete the form until the expression is made true (X + Y + Z = 100). To do so, you need to create two new required text fields, assign them the exact same data label, give them conflicting validations, and make them conditional to the formula field (make these fields appear when trigger = 0).
In order to create conflicting validations, you need to select Custom Validation and use a regex pattern. The example below only accepts digits 0-4 in the first field and digits 5-9 in the second.
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