For a mesophyte growing in a moist soil, water potential of its root sap is -3 atm, then how to find the water potential of moist soil and leaf respectively?
I don't know if we can calculate the exact values. The only thing I can say is that the potential of the soil will be less negative than -3 while that of the leaf will be more negative.
You know that water travels from higher potential to lower potential. So, it will travel from a region of less negative water potential to a region of more negative water potential.