In the UK, in a normal situation with a 3 (or more) person team, the SM doesn't call the show. That is the role of the Deputy Stage Manager.
As an SM on shows like this, I would start the show, watch any clearances, cue entrances, etc. I suppose you could think of it as an ASM, just with a different priority for tasks, and more responsibility if anything adverse were to happen.
Hope this helps - let me know if you have any questions!
C