Here is the definitive source:

Exceptions to the General Residence Conditions - from the SAAS (Student Awards Agency Scotland).

General residence conditions

In general, to be eligible to apply for support, you must meet the following conditions:

Be ordinarily resident in the United Kingdom, the Channel Islands or the Isle of Man for the three years immediately before the relevant date (the first day of the first academic year of the course). If you fall short of being ordinarily resident in the UK for the three year qualifying period, even by one day, you will not be eligible to apply for support

If you are not a UK or other EU national, you must also have ‘settled status’ in the UK (as set out in the Immigration Act 1971) on the relevant date. If you have not been living in the UK, you may still qualify depending on your circumstances.

You must also be ordinarily resident in Scotland on the relevant date. We will not treat you as being ordinarily resident in Scotland if your main purpose in coming here has been to receive full-time education and that you would have otherwise been living elsewhere.

If you are not eligible for support under the general residence conditions, you may still be eligible to apply to us for support under one of the exceptions listed below.

I'm not going to copy/paste the whole of the document, but the three year rule is quite clearly explained. I was wrong, however, about the date requirement. There are set dates by which you must have been ordinarily resident for three years in order to qualify as a 'home student'.