These small breakers are usually the thermal type. They can gradually reduce their tripping current
over time. Also, high heat in the genset compartment can make them trip. Vibration is also something that can cause degradation.
Recommend you get a spare breaker and keep it on hand then I would just continue to watch the old one. I might even try to measure the current
in the old breaker to see if it is close to 3A (like 2.5A). But it isn't always easy to make that measurement. If it trips again and you are on site check the temperature around the genset. It could be some air passage
has become blocked with debris or possibly some insect has built a nest. If the air temperature is normal then I would replace the breaker and hold the old one as a spare. If the new one trips then you have to do some more troubleshooting.