Weighing - Find one/more fake coins

I have scales, without any known weights, so that I can only compare : "heavier", "lighter", "same weight".
I also have 13 identical coins, but one is fake, of a different weight.
Find the fake coin in only 3 weighings.
Solution

I have now 7 coins but two fakes are lighter. Find the two fakes.
Solution

Details

 

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