You're making a binary adder. Try this...
Two black boxes for input with one input remaining:
7 6 5 4 3 2 1
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| H L | | H L | |
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Take the two low outputs and the remaining input (1) and feed them to another black box:
L L 1
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-------
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| C L |
-------
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The low output from this black box will be the low bit of the result. The high output is the carry bit. Feed this carry bit along with the high bits from the first two black boxes into the fourth black box:
H H C L
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| H M | |
------- |
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The result should be the number of "on" bits in the input expressed in binary by the High, Middle and Low bits.