First, I'd contact Diluvio and ask what amps they recommend or recommend against.
I don't much care what mono power amps are out there, so I can't make any recommendations at all. Any mono amp I'd use would have a 70 volt output. One thing is that the mono amp requires putting all three speakers in series or in parallel. In series, you'd get a total of about 2/3 of the amp's rated output (guessing -- it's too late to do any math at all), and that would be split among three speakers. So a hundred watt (at 8 ohms) amp would give you about 22 watts per speaker. Any problem with a speaker that would affect its impedance would affect all three speakers.

Is the amp going to be protected indoors? Use three channels of a four channel amp, something beefy like a crown. Three amp channels that total 100 watts (sum of the three outputs into 4 ohms) would give you about 33 watts per channel.