If candidate is proven effective, frontline health and social care workers could be vaccinated next summer.
Deal struck in principle between UK Government and specialty vaccine company.
US-based Scots scientist Dr Kate Broderick hopes preventative jab can be tested on people in April.
The two new patients are based in Ayrshire and the Grampian area respectively, while the first was in Tayside.
Ashlee Easton, 13, who has battled cancer for seven years, hopes to travel to the US for potentially life-saving treatment.