I was at a conference last year and Three Horizons was mentioned by one of the speakers. I had not come across the concept. I have since looked into it more and found it a real useful way of thinking about the future. I have developed a Miro board that centralises some resources and templates to map out a Three Horizons landscape.
The link is Three Horizons. I have also attached a PDF template and also an overview of Agency vs Uncertainty which helps position when you should consider using Three Horizons approach.