You can do this with the blending tool. Looks that the designer used 2 overlapping shape blends.
Below a very rough example (blending takes some time to tweek to get it look perfect – often you have to expand and fix some issues manually after blending) Also top right of your example has got a light shadow that helps the 3d effect...
You start out with a white diamond shape with a black outline. Then you make two copies of it, scale the width down so it's very narrow like at the bottom and top. The shape at the bottom should be in the foreground and the one at the top in the background. Last step is to blend the shapes and edit the blend path to make it round.