Put the flour and water in the bowl and mix for 4 minutes at low speed.
Remove the bowl from the machine and cover it with a damp cloth.
Leave to rest for 1 hour, then add the sourdough starter, fresh yeast, and salt.
Knead with the dough hook for 4 minutes at low speed, then for 7 minutes at high speed.
Put the flour on a work surface or in a mixing bowl and make a large well in the center.
Pour in two-thirds of the water and mix until all the flour has been incorporated.
Leave to rest for 1 hour under a damp cloth, then incorporate the rest of the water, the sourdough starter, fresh yeast, and salt.
Knead the dough until it becomes smooth and elastic.
Shape into a ball and cover with a damp cloth.
Let rise for 1 hour 30 minutes. It will have increased in volume by the end of the rising time.
Dust the work surface. Divide the dough into 3 equal pieces.
Fold each piece over on itself, pulling gently to stretch into a longish log.
Cover with a damp cloth and leave to rest for 30 minutes.
Working with 1 piece of dough at a time, use the palm of your hand to flatten it gently.
With the long side facing you, fold in a third towards the center and press along the edge with your fingertips.
Swivel the dough 180 degrees.
Fold in the other long edge so that it overlaps in the center and press with the heel of your hand.
Fold one half on top of the other, and seal the edges together with the heel of your hand.
With lightly floured hands, roll the baguette out to 21 inches (55 cm) long, then pinch each end into a point.
Shape the other 2 baguettes the same way.
Carefully lift the baguettes onto a lightly floured baker’s cloth, seams underneath.
Separate them by making folds in the cloth.
Cover with a damp cloth and leave to proof for 1 hour 40 minutes, by which time the baguettes will have increased in volume.
Place a baking sheet on the bottom shelf of the oven and preheat to 450°f (230°c).
Gently place the baguettes, seam down, on another baking sheet lined with parchment (baking) paper.
Dust with flour and make 4 evenly spaced oblique slashes along the length of each baguette.
Just before putting the baguettes in the oven, pour 50 g (scant 1/4 cup) of water onto the preheated baking sheet.
Bake for 20 minutes. Remove from the oven and leave to cool on a wire rack.
