I'll assume your transducers are 4 ohms. Resistance in series adds up so putting the transducers in series makes a total of 12 ohms. This will decrease the total power available from the amp. The transducers will each take a portion of the power. An equal amount if they are the same impedance.
You need the following formulas:
Voltage = square root of power times impedance
Current = voltage divided by impedance
power = current squared times impedance
To figure it out get the rated power of the amp into 4 or 8 ohms. Take this power times the ohms and then the square root of that. This will give you the voltage rating of the amp. So if you have 100 watts into 8 ohms it will be the square root of 800 = 28 volts. 200 watts into 4 ohms gives the same answer of 28 volts.
Since you now have 12 ohms the maximum current will be 28V divided by 12 ohms = 2.3 amps. Current in series is the same so each will have 2.3 amps. Take the square of 2.3 times 4 ohms = (2.3 x 2.3 x 4) = about 22 watts for each transducer or a maximum from the amp of 66 watts.
If they are different impedances do the same calculations and you will see that the higher impedance gets more power. With one 8 and two 4 ohm you would have a total of 16. The same 28 divided by 16 = 1.8 amps. The 8 ohm would get (1.8 x 1.8 x 8) = 25 watts and each 4 ohm would get (1.8 x 1.8 x 4) = 12.5 watts.
