They don't take 2-3 weeks of manufacturing from start to finish. They manufacture vast quantities in a season to begin with and they decide on the models, colors and quantities way before a season(aka collection) and their suppliers start manufacturing accordingly. What they do is instead of starting a season with every piece they have in their collection, they have time frames that are 2-3 weeks and they show different pieces from the collection every 2-3 weeks. That's why if you go to Zara every 2-3 weeks you see something different. That is called fast fashion.
How they manage do it is they either store their products in a warehouse and display them at the stores on predecided dates. Or they arrange the goods' lead times according to predetermined time frames with their manufacturers before the production starts. They work way in advance. I'm sure they're already almost done with their Winter 2016 collection and production scheduling as I write this.
How they manage to pull this off is thanks to their massive turnover rates, size and having subcontracted big manufacturers all over the world alongside a strong woking capital.