What is the purpose of peer evaluation of teaching?
I'm attending a similar program at my university (also UK based), and I was observed by a senior member of my faculty, and by another attendee of the same program. 

The main point was to initiate a reflective process of our teaching. Hence, the observation was not an evaluation, but simply the collection of some pieces of evidence, that described our lecture. Our actual assignment was not the observation in itself, but how we reflect upon the evidence, by describing how we feel about it, how we analyse it, what conclusions do we make about it, and what are the next steps we decide to engage on. 

Interestingly, I've collected different types of evidence from my two observers: the senior member of my faculty made some observations in particular related to the content of my lecture, while the observer who was on the same program made some observations related to some of the techniques taught in the program that I used during the lecture. In both cases, all observations were really useful to start the reflective process. I can only assume this is the rationale for you to have three different observers.