Re: New System New Problem
You might try disconnecting one of the speakers to see if it is the problem for sure. If that works then some impedance matching transformers may work for you. I have used impedance matching volume controls for this type application just for the impedance matching with the controls set all the way up. Finding 300w units could be a problem.
The other possibility is the single amp may not deliver enough power to drive all three speakers. If that is the case you would need a larger amp. You do have the amp bridged? If not that would probably help a lot.
What is the wire size and length of the run?
Finally, there could be a lot of losses through the run if you are using low impedance output from your amp, 2 to 16 ohms say. Using a 70v line would probably correct that issue. If your amp has a 70v output, then get some 70v transformers for the speakers. If your amp does not have a 70v output then you would need a 8-16 ohm to 70v transformer for the amp end. I'm not sure it would be worth this option though because high power units are expensive.