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.