If i've been given the next three years with mental health support and a social worker can i declare a disability at work?

Depends: The word "disability" usually refers a to situation, where due to a physical or mental condition, one is unable to perform their usual work-related duties. In matters of employment, one would turn to their human resources department to understand what if any benefits they may be due. A diagnosis, or in this case, a set of treatment recommendations, does not automatically mean one is disabled.

Speak with HR: I'm not quite sure what you are asking. You would need to speak to your HR department regarding FMLA or short-term State disability. If you seek to apply for SSI/SSDI, which are federal benefits, you may do so online. This process takes a few months to complete, & will require statements from you, providers, & family if possible. Diagnosis doesn't determine disability, functioning level does.