Insoluble salts are compounds that do not dissolve in water. Option 1 is incorrect because chlorides of Fe (iron) and Mn (manganese) are actually soluble in water. Option 2 is also incorrect because nitrates of Ag (silver) and Pb (lead) are soluble in water. Option 4 is incorrect as well because phosphates of Na (sodium) and NH4 (ammonium) are also soluble in water.

Option 3 is the correct answer because carbonates of Pb (lead) and Cu (copper) are insoluble in water. When a compound is insoluble in water, it means that it does not dissociate or break up into ions in water. Instead, it remains in its original solid form. In the case of carbonates, they typically react with water to form hydrogencarbonate ions and release carbon dioxide gas. However, lead carbonate and copper carbonate are exceptions and do not readily dissolve in water. Therefore, they are considered insoluble salts.

In summary, only option 3 - carbonates of Pb and Cu - is correct because these salts are insoluble in water.