In the first step, solute particles must be separated from one another. The more tightly they are bound togther, the more endothermic this step will be (ΔH₁ = +). In three of the compounds used in this experiment there is a clear progression of ionic bond strength or character if we use electronegativity difference as a rough guide.

Does that mean the enthalpy change for NaCl dissolving will be more positive than for the other two compounds? It could mean that if step 1 is a larger enthalpy change than step 3 (which is always exothermic).