To simply lower the volume of the tweeter in a circuit that includes only the tweeter (like a tweeter being used on it's own channel), you can use a resistor in series. For a reduction of -3dB for example, you would use a resistor of 3.3ohm assuming an 8ohm nominal speaker. The amp would see a total nominal resisitance of 11.3ohm.
This, however, won't work if you've already taken into account an 8ohm nominal load if you've already configured the crossover. By padding the tweeter with a resistor, the rest of the parallel circuit now sees a nominal 11.3ohm load.
To fix this, you need to form a fixed L-pad with a parallel shunt resistor in place. You can go to this website to calculate you values:
L pad calculator - attenuation dB damping impedance decibel loudspeaker speaker voltage divider - sengpielaudio Sengpiel Berlin
To lower the volume of an 8ohm tweeter by -3dB (for example) but keeping the rest of the "upstream" crossover components working as they should, you need to insert a ~2.3ohm resistor in series with the speaker, and a ~19.3ohm resistor in parallel with the tweeter.
I will also note that you have other problems in the way you assume crossovers work. A nominal 8ohm tweeter wired in parallel with two 8ohm mids will not produce anywhere near a 2.3ohm nominal load at any frequency. The lowest dip you would see in impedance
would likely be about an octave higher than the fs of the mids (or the fb of the enclosure, assuming a sealed design and not a free-air speaker) after the Q effects taper off and you're left with a more resistive load. It would be safe to assume that around this point the nominal impedance of each driver will be paralleled to produce a final load of around 4ohm. You can model the drivers and get an idea of the impedance minimum (Zmin), or run your own impedance sweeps to know for sure though.