Matlab Three Phase System Model
The issue is the final display not the actual data.

If you are using a fixed step solver then use a small timestep. If however you are using a variable timestep then you need to inform the solver to render/calculate additional datapoints. 

Simulink is aware what hte actual signal is as it is an integration engine and is sensitive to zero-crossing. In rendering a plot it has deemed this is all the information that is needed to represent the data.

This does produce unexpected results.

Consider a 100Hz sinewave as below. Not very sinus

What is needed 



What is needed is to change the default refinement factor IF you are using a continuous solver. 



The defaults is 1 so change it to 100



You now have the expected waveform. What this does is requests the solver to generate additional points between the datapoints it needs to realise the waveform. 

A good explanation is found at "guy on Simulink" blog. https://blogs.mathworks.com/simulink/2009/07/14/refining-the-output-of-a-simulation/