Is there any way to scale the 3D sketch proportionately in all 3 directions?
No you can't scale a 3d sketch directly but create a surface using your original 3d sketch. Now scale that surface and start a new 3d sketch and using convert entities convert one edge into your final scaled up 3d sketch.
No like Deepak said, but you can scale the 2D sketch inside of the sketcher.
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