You have 3 substances that are all white solids, ammonium chloride (NH4Cl), sodium chloride (NaCl), and magnesium carbonate (MgCO3). First of all, the sodium chloride and ammonium chloride are both highly water soluble whereas the magnesium carbonate is only very sparingly soluble in water. So taking the whole mixture and putting it in water will solubliize the NaCl and NH4Cl. Run the suspension through a filter to obtain the undissolved MgCO3. Now the NH4Cl has the ammonium cation (NH4+) that can be treated with base to give ammonia (NH3). So treat the aqueous solution with sodium hydroxide (NaOH) to convert the NH4+ to NH3. Ammonia (NH3) normally exists as a gas, so warm the aqueous solution to cause the ammonia to evaporate from the water as a gas. Collect the gas and pass it through some HCl to regenerate the ammonium chloride (NH4Cl). Finally, boil away the water to isolate the sodium chloride (NaCl).