...Very difficult to see any practical function without further analysis...
A thought occurred to me some time ago as to a possible use for such an object.
Ancient Weights and Measures?
Dividing something - for example, a sack of grain - into two equal parts is simple enough - you use a simple balance scale.
However, dividing a sack of grain into THREE equal parts is a bit more tricky. This metasiltstone bowl might represent a means by which such could have been easily achieved.
Imagine the artifact is suspended at its centre with a cord. Now imagine the bowl filled with water. The water will create a level (like a spirit level). Mark the water level on the inside of the bowl. Finally, imagine three cloth sacks with a strap placed inside and looped around each of the three lobes ('leafs'), the cloth sacks being suspended under the bowl.
Even if the three sacks are all of different weights these can be equalised by adding small pebbles to the appropriate sack until the water in the bowl is again level. Now fill each of the three sacks with the grain, ensuring the water in the bowl remains level. When you have finished, each of the three sacks will have the same quantity (weight) of grain.
Such a weights and measure scale would be 50% more efficient than a simple balance scale.
Just a thought.