If I had 3 iron provinces I'd use one for Militia Line and Spearmen, one for swordsmen and one for the royal cavalry line. You can probably add in a few multi's because some units require multi tech lines, such as Mounted Seargeants and Kwarzmian Cavalry need spear and horse buildings.
Then again if I ever got some iron provinces for Turks they's get one for The Jannisary Heavies (spearline), one for the Hybrid archers/Futtuwa's/Jannisary Infantry/Hashishin (Bow and swordline) and one for the Ghulam cavalry and Kwarazmanians.
All depends really.

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