Can somebody tell me if this is 3 complete sentences. Agriculture was key to the development of modern "civilization" because by being able to grow food rather than getting it through trade or hunting, people were able to stay in one place instead of having to be where the food was. This also led to jobs because people became highly skilled in growing crops.

Technically that would be two.

Explanation:

Although if you manage to reword that first sentence, you might be able to split it into two sentences

this forms a narrow, steep sided inlet (sometimes deeper than 1300 metres) connected to the sea. the terminal moraine pushed down the valley by the glacier is left underwater at the fjord’s entrance, causing the water at the neck of the fjord to be shallower than the main body of the fjord behind it.