30L mountaineering bag, is it suitable for hiking in the rain? Is it big enough? Two SLR cameras and three lenses.

The size of the backpack depends on the length of time.

If you live in the local area and have other travel packaging clothes and daily necessities, then a small bag of 20L or even more than ten liters is enough for a single day (provided that there is no telephoto head and big fuselage, I am afraid it will take 20-25L if there is one).

Specifically, tell me your estimated travel time, whether there are other bags, what is the camera lens (it is convenient to estimate and mention, after all, the gap between 50 18 and 70200 is too big), and the approximate departure time, because it is necessary to estimate the amount of clothes according to the weather in advance (this actually depends on gender ...) ... and whether there are other people passing by, and whether you are the only one with the camera (in this case, others) Didn't you finally bring a tripod up the mountain?