You can use water to instantly separate the three. Sawdust floats in water, while sand does not. The salt will dissolve in the water.
To really separate the three, collect the sawdust first. Dry the sawdust and water combination to get the sawdust. Then make sure the salt is dissolved in water. Separate the sand from the water. This leaves you sand. To separate the salt from the water, make the water evaporate.