In my HOMER simulation, three diesel generators run sometimes even if the load is smaller than two diesel generators. My system has three 150 kW generators, 120 kW PV, 100 kW wind, 500 kWh of batteries, and load between 80 and 310 kW.

If the operating reserve is 10% (the default), and the load is 290 kW, the total available capacity must be 319 kW (290 * 1.10) and HOMER would turn on a third generator to meet this requirement. An operating reserve of 25% of PV output and 50% of wind output is also standard. So if you had 290 kW load, 80 kW of wind, and 100 kW of PV, you would still need two generators turned on:
Required operating capacity = 290kW * 1.1 + 80kW * 0.5 + 100kW * 0.25 = 384 kW
Supplied operating capacity = 80 kW + 100 kW + 150 kW *2 = 480 kW

With only one generator operating, there would only be 330 kW of capacity, which would not supply the 384 kW requirement, even though at first glance this is more than 290 kW.

The PV and WT contribute to both the operating capacity and required operating reserve. This is because they contribute power, but also contribute more variability. The load profile is like the average power demand for each time step, and the operating capacity is the peak power for each time step. The system must have the ability to support the peaks, but generator fuel usage, battery discharging, etc, are based on the average power.

So, imagine you have the operating reserve include 50% of wind power output (screenshot PNG attached), and a time step where the wind output is 10 kW. The wind production increases the operating capacity by 10 kW, and it increases the required operating reserve by 5 kW. The wind power produced 10 kW on average for this time step, but at some times it produced as little as 5 kW (and at other times it produced more than 10 kW). If you have a 10 kW load, you need batteries, a flywheel, an operating generator, or something else to support the system stability, even if no additional power is needed on average. If you have a 5 kW load, the wind power would be enough to support this load alone (and the required operating capacity). If the load is supported by the generator, addition wind or PV power will always help meet the required capacity -- it will never hurt, unless you set the operating reserve percentage (as in the attached PNG) to greater than 100%.

I have attached three PDFs from our help documentation.