A small speaker rated at 12OHM / 50W (as a woofer)
A small TV speaker rated at 16OHM / 2.5W (mid range)
and a very tiny speaker rated at 3OHM / 3W (tweeter)
Those are all rather "odd". Most speakers are 4 or 8 Ohms. Some car woofers are 2 or even 1 Ohm.
Just some general thoughts & information...
Impedance is similar to resistance and both are measured in Ohms, except impedance is frequency dependent* and you can get phase-shifts. So I'm going to simplify by talking about resistance.
When you put resistances in parallel the total resistance drops. With a 3-Ohm speaker the total will be LESS than 3-Ohms.
If you put two 8-Ohm speakers in parallel you get 4-Ohms.
In series they simply sum, so two 8-Ohm speakers in series is 16-Ohms. With 4 or 16 matched speakers you can make a series-parallel combination with the same impedance as one speaker. (But you can't put a woofer & tweeter in series and if you put them in series along with a capacitor it will block the low frequencies to both.)
The formula for parallel resistors is: 1/ (1/R1) + (1/R2) + (1/R3).... 1/(1/12) + (1/16) + (1/3) = 2.09 Ohms.
If you just have two resistors there is a simpler formula: (R1 x R2)/R1 + R2)
Resistance (and impedance) is "the resistance to current flow". Lower resistance means more current flow (given the same voltage) The relationship between voltage, current, and resistance is defined by
Ohm's Law.
There is a plumbing analogy where water pressure is voltage, water flow is current flow, and a skinny pipe (or a valve turned partially-on) is high-resistance and a fat pipe is low resistance.
...But when you cut a water pipe you get zero resistance and water flows-out all over the place. When you cut a wire you get infinite resistance and no current flows. And with no water-resistance nothing burns-up.
Usually the voltage is controlled or constant so current depends on resistance. Here in the U.S., there is 120VAC at the power outlet and that voltage is always there (unless there is a switch to turn it off). If you plug-in a 100W light bulb, a little less than 1 Amp flows. A toaster or hair drier might "draw" 12 Amps. If you plug-in (and turn-on) two toasters you'll probably blow a breaker and then voltage drops to zero.
If your speaker impedance is too low you might fry your amplifier, or if you are lucky it may just go into temporary thermal shut-down.
Power (Wattage) is calculated as Voltage X Current so
if you cut the speaker impedance in half you can get twice current and twice the power, assuming the amplifier's power supply can supply the current and assuming the amplifier doesn't burn-up.
Higher impedance won't hurt the amplifier but you get less power.
A
proper crossover distributes the voltage & current so you can use an 8-Ohm woofer and an 8-Ohm tweeter (and an 8-Ohm midrange) you have an 8-Ohm speaker.
Just a capacitor to the tweeter means that at high frequencies, current flows through the woofer and the tweeter so it would be 4-Ohms at high frequencies and 8-Ohms at lower frequencies. You can usually "get away" with that (with an amplifier rated for 8-Ohms) because the high frequencies are usually "weaker" (less power and current) and to some extent the woofer's voice coil acts like an inductor which has higher impedance (inductive reactance) at high frequencies.
* Capacitors have capacitive reactance which is also Ohms and an impedance component. As you may know, capacitive reactance is inversely proportional to frequency and that's why they "block" low frequencies when in series, and they bock DC which is "zero Hz".
