It's the employer's statement attached to certain immigration applications, like work-based nulla osta requests. It sets out the employment offer, the contract terms, and the employer's details.
Without a properly filled S3, the work-based application is incomplete. The employer signs and submits it, usually with help from a labour consultant or patronato.
Each Decreto Flussi cycle can use slightly tweaked forms. Always download the latest version from the official portal before filing.
Official source
Ministero dell'Interno
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