and not that easy to answer. All three brands have offers in different price levels. I will try.
Lets compare the price level € 4-10K, steel cases, Leman line from Blancpain, Master line from Jaeger and GS.
In this case I would say Blancpain is superior (cases, crowns, dials, indexes, hands, clasps, ...). The cases of Jaeger and GS are equal, but the crowns from Jaeger are superiour GS.
Observation excluding the movements and the design - based on personal experiences/impressions.