Of course, you aren’t limited to just three layers, and you should adjust the number of layers you use according to the weather and the level of exercise you’re doing.
For example, you may want an extra layer of insulation between the mid-layer and outer shell in extra cold weather. Similarly, if it’s warm and wet, you can leave out the mid-layer and just use a base layer and outer layer.
You can also use different types of tops and jackets for each layer (apart from base layers). So, you can use a softshell jacket as your mid-layer, but as they offer a decent level of breathability and waterproofing, they can sometimes also be used as an outer layer.
The type of activity can also affect this. If you’re climbing then you don’t want to be encumbered by thick layers, so more lightweight clothing is preferable, while when hiking this is less of an issue.