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Assuming you want to keep the first two sketches, you can Project geometry from Sketch1 and Sketch2 into Sketch3. This will create a link - changes made to projected geometry in Sketch1 and Sketch2 will be modified in Sketch3.
Or, just Project geometry from Sketch1 into Sketch2.
If you want a new sketch but no longer want the two existing sketches, you can Copy & Paste the geometry in to the new Sketch, and then delete the old ones.