Hi Naomi, are you referring to an actual sourdough bread dough, or sourdough starter? A bread dough that has been left out for 3 days would likely be very over-fermented and may not recover well, even with additional flour. However, if you're talking about sourdough starter that has been left out, then, yes, I would encourage you to discard all but a small portion of the remaining starter and feed it twice daily at room temperature according to this feeding routine, until it is doubling within 6-8 hours and has a pleasant aroma. In future, I would also recommend covering your starter more securely with plastic wrap or a lid, so the surface doesn't dry out. The reason we say to "cover loosely" isn't because the starter needs airflow in order to ferment properly (it doesn't), but because fermentation gases can build up in a tightly lidded container and cause the lid to pop off. It's fine to cover your container with plastic wrap (which will flex). A lid is also perfectly okay, as long as there is room for the fermentation gases to collect above the starter level and you open the jar now and then to vent the gases.  

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