Need more information -- what are your actual duties?(for example if your narcolepsy affects the safety of you running a machine, that's more of an issue than if you are an accountant sitting at a desk staring at a computer and spreadsheets).
Has your doctor been able to classify your issues as an ADA defined disability and/or in need of time off to deal with the issue? Have you missed more than 3 consecutive days of work for this condition -- and does it affect major life functions? The employer can refuse to let you return to work if you are not 100% fit for duty, but can not require you to take off more time than you need (subject to some possible scheduling issues) for FMLA leave. Which might seem contradictory, but not if you aren't 100% fit for duty (and that depends on your duties as I mentioned above)
Under FMLA, employers can require you to give as much notice as possible for need for timeoff. Here is what the DOL states on intermittent leave " Employees may not be required to take more FMLA leave than necessary to address the circumstances that cause the need for leave. Employers may account for FMLA leave in the shortest period of time that their payroll systems use, provided it is one hour or less. (See CFR Section 825-205)
Employees needing intermittent/reduced schedule leave for foreseeable medical treatment must work with their employers to schedule the leave so as not to unduly disrupt the employer's operations, subject to the approval of the employee's health care provider. In such cases, the employer may transfer the employee temporarily to an alternative job with equivalent pay and benefits that accommodates recurring periods of leave better than the employee's regular job."
Has your doctor documented the fact that your condition could occur at any time? Have they documented how often the attacks are happening/expected to happen? If you are going outside of what has been documented, you need to get updated documentation to your employer ASAP. They are only required to go by what they have and can request further information if needed.
How much time have you actually missed in those 3 months?
Much of what they are saying could possibly be misunderstood, or they could possibly be trying to terminate you for having the condition (both of which could be very stupid on their part) or you could be misunderstanding the why behind what they are saying (example what I have written above)

