I have been on disability for the last 3 years as a result of bullying and harassment at work for which I have filed a case against my employers.I am on certified sick leave.
My employers are threatening to terminate my employment even though their independent medical doctor has deemed me unfit to return to work.

Can they fire me?

I am not a lawyer.This information is only opinion.

After 3 years,how effective can you be for this company?Technology has changed at least 9fold in that time,your skills may or may not be in a field which is directly related to that,but you still have a long term problem,one which your employer sees is not going to be resolved with you on their payroll.

Perhaps a lawyer to lawyer discussion is in order here,yours talking to theirs and finding the reason for their want to let you go(perhaps they want to settle this dispute and for some reason,if you are on the payroll,it could become a difficult settlement)

I sense there is a lot of information missing from this simple post you have,and further information might compromise your 'case'.

Look at both sides of this situation and decide what is in your best interest, if you have any control,make things in your favor before any changes take effect.

Again,I am not a lawyer,not a legal adviser,only someone with an opinion.

I find nothing that says your job has to be held open until you are medically able to return to work. If that open job is causing your employer's business to suffer, yes, you can be terminated by virtue of the lengthy leave of absence.

I don't know if you have a union or employment contract but I am seeing employment contracts here which advise that if you are out "sick" for more than 365 days you will be terminated.

Have you consulted with an Attorney? It might be a very good idea.

Thanks for your responses.
Yes, I do have a lawyer and barrister working on this case and await their advice here.
My employers have written to me despite having a legal representative and asked me to respond to their letter regarding my wishes to