This has not been an issue previously: I searched a destination this morning and saved the result to 'My places'. Choosing 'Navigate', one route alone displayed. I searched a local postcode, which has returned three choices always, and selected 'Navigate' directly. One result again.
I have Premium Features, and 'Alternative Routes' is shown as 'In purchased pack'.
'Alternative routes' is checked in Navigation>Advanced.
I am running version 3.1.120 on my Moto G 4G Android 'phone.
Previous navigation calculations have resulted in three choices to this destination, as I wrote in my original post.
I have changed vehicle settings recently to reflect more appropriate speeds of different classes of road. I would not expect those changes to prevent choices. I have not excluded any road classes except 'Other road'.
I had changed 'Preference' percentages in 'Road Restrictions' to decrease according to the roads hierarchy, in addition to changing the speeds themselves. Returning 'Preference' to 50% in all cases resulted in three routes appearing once again upon calculating navigation.
It leaves me to wonder further how to use 'Percentage' effectively?