Genos1 power usage is only about 30W, Genos 2 should be similar. JBL305 max power usage is 127W according to rear power panel on unit, actual power usage should be less. That's 300W to cover the 3. Total power will depend on your attachments, can you specify?
The next step is how much battery power you will need. One way to determine this is to place a (cheap) power monitor on the mains power feed to all your gig equipment and measure the total power used for an entire performance, including idle time, in kilo watt hours (kWh). If your performance lasted 2 hours and used 0.3 kWh of power that's 300 watt hours / 2 hours = 150 watts average power but the inverter has to cover the peak power of 300W + attachments. The battery has to provide 300 watt hours of energy, for a 12 volt battery you would need 300/12 = 25 Amp Hour capacity but you have to allow for inverter and battery efficiency. At 85% efficiency actual battery capacity required would be 25/0.85 = 30AH.
Battery/inverter efficiency will depend on the battery type, lead acid battery efficiency depends on load and could be less than 75%. Lithium battery efficiency is much less dependent on load and battery/inverter efficiency can be 90%.