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Delete the other two circles, we will first make the whole shape on the top and later array it over to the other two corners.
Hide the rectangle and our small 15 untis long line.
Next, go to 'curve/line/tangent to 2 curves'.
Click near the tangent point on the first circle, then do the same on the other circle for the second point.
Do the same on the other side.
Select all these curves and use trim to cut away the inner parts. Join these curves into a single curve.
Use polar array to place this shape in the other 2 places.
Click on the fillet curves button.
Click on the radius option and enter 3. Also make sure you set the join option to yes.
Fillet all the sharp corners by clicking on the two lines that make up a corner.
Again draw a triangle with and edge length of 18.5 units.
Place it in the center.
Fillet the corners with a radius of 3.
This is the final shape.
Experiment with some extrusions and boolean operations to make a 3d shape out of it.