"I don't think that it is a natural process."
Translation:Не думаю, что это естественный процесс.
естественный is, well, "natural" in the sense that it occurs naturally and/or spontaneously without requiring a certain intervention to make it happen. If a person's behaviour is natural and spontaneous we also use естественный to describe that (e.g., естественное поведение)
натуральный usually refers to natural origin of materials, substances, or food. We also have натуральные числа (ℕ) and expressions like "в натуральную величину" (full-scale, 1:1).
природный is an adjective from природа "nature" and means "natural" in the sense of "as found in nature". We also use it for something "innate", which has a lot to do with a person's "nature" also translating to "природа" in Russian.
For obvious reasons, естественный is the most used adjective of the three, which is not to say the other two are optional. These adjectives are rarely interchangeable simply because they describe different things. However, some meanings pop up much more often in real life conversations—and when you have to express them, you use the appropriate word.