What's the difference between these 3? What is the difference between satipatthana, samatha and anapanasati?  When I read about them they all seem to be saying the same thing so how are they different?
Those three terms do have a lot of similarities when we apply them.

Anapanasati is a broad term translated as "mindfulness of breathing", it can refer to any type of breath-meditation where we apply mindfulness. In the anapanasati sutta it mentions being mindful of the body while you are breathing, therefore you don't have to be mindful of the breath directly.

Samatha means a calm state so samatha meditation is a type of meditation where we try to create calm. Often being aware of the breath is a way to create calm so this can appear similar to anapanasati.

The word satipathana means a foundation for mindfulness. This is a type of meditation where to aim to practice mindfulness of anything we choose. The satipathana sutta uses the body and feelings as examples of things we can be mindful of. If you don't have much calm then it can be good to be mindful of the breath because it is quite stable. This can be a type of anapanasati meditation; also notice how it is linked to creating calm (samatha meditation). 

Those three types of meditation overlap a lot and sometimes we could use more than one of those words to describe what we are doing.