Been trying to achieve this effect, how does one do it?
We do not know how transparent the presented shapes are. That makes blending mode speculations a little unsure. If you want some certain colors, you can input them directly:
There are three overlapping blue shapes. The overlapping zones are separated by applying Pathfinder panel > Divide and ungrouping.
The black strokes are removed. The zones have got their fill colors with the color picker from your example. In the right there's added on the top a rectangle with partially transparent gradient fill.
If you are going to export the result as PNG, the strokes would be useful. They should have the same colors as the fill colors. That's because otherwise the seams in the export easily could have 1 pixel wide transparent zone due the antialiasing in the rasterization. The strokes would cover the gaps.