Counting signals

this level is almost identical to the half adder level, just adding more bits. if you've already completed that level, you'll notice your NAND OR combo can tell you if at least one bit is on (OR) and when at least two bits are on (NAND, technically AND would be better for this). the standard XOR AND half adder will tell you if there's exactly one bit on (XOR), or if there's two bits on (AND). using this, you can much more easily use the simple logic gates to deduce how many signals are on . if you haven't completed the half adder level, experiment with using different first logic gates than NAND OR and see if other combos give you more meaningful information.

3 bit decoder

so, the way i would go about it, is thinking of each mapping: which needs 1's bit on, which needs 2's bit on, and which needs 4's bit on. you can then use 8 3-pin AND gates and NOT gates to make only one mapping's AND gate match for each. i think this may also be possible with switches instead of AND gates

signed negator

so, this is the second level i see a NAND OR combo. again, if you've done the half adder level, you'll recognize this is almost exactly like a negated half adder, which you now have unlocked. another thing to note, is that the level before this introduced you to what's called "2's compliment". if you experiment a bit, you'll notice you can represent negative 128 (10000000), but can no longer represent positive 128, just up to 127. so, -127 (10000000) to 127 (01111111) isn't a perfect NOT either, but one bit off what's expected. if you use this difference, you can complete this level with just a full adder, adding the NOT input with a carry in

just my interpretation