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This can either be done through mask - by forcing the input to be a 3 digit number and using sub label to explain that leading zeros should be added, or to use the conditions to add the zeros to the value entered in some other field.
You can see this in a quick demo here: https://form.jotform.com/60334978432965
You can see how both are done by cloning the form to your own account following steps here: How to Clone an Existing Form from a URL or by asking more here and we would be happy to explain each.