What I want to know is ... why is there a mouse running across the palace floor in the beginning of episode 3? At first I thought it would be significant for the storyline in this episode, but no. I just seems very odd with that mouse

I was delighted to see that mouse.

It was a highlight of the season so far for me.

I immediately thought of two things: the cute mouse in the Poirot episode 'Hickory, Dickory, Dock', and the possibility the mouse was a symbol of the underlying decay of the royal properties which have gilt decoration on the top but rotting pipes and timbers underneath riddled with vermin.
With a more wide awake and active brain this morning I am wondering whether the mouse might symbolise more. Perhaps it suggests the whole system of the monarchy is a pretty construct balanced precariously on a rotting and unstable foundation. Maybe it is a Cinderella reference. Maybe a reference to the actual words of 'Hickory, Dickory, Dock' and Diana is the mouse. Or maybe someone just thought it would be a good idea to put a mouse in that scene. Perhaps a bit of all the above. Whatever the reason, that mouse might be my favourite bit of the season, because I am not finding the season to be particularly enjoyable.