I've 3 sets of stress strain points which ones do i use to calculate youngs modulus in stress strain curve?

You should be able to use them all. They should all give the same result unless plastic work has been done on the material. That changes everything.

You'll have to plot each of them on a different curve, and obtain Youngs modulus. This will give you 3 values: E1, E2, E3. By getting an average of this 3, you get an approximate and more accurate value from you experiment. Hope that helps!

plot the stress against the strains,the y -axis being for stresses and the x axis being for strains.Then see the behavior of the curve or line that you have plotted.If all the three points form a straight line then your data for the that particular material is in the elastic range,it means that both points have the same modulus of elasticity,which you can get by dividing the stress by the strain for each point.You will find that you will get almost the same Modulus at every point.It is more like finding the gradient of the line that you have plotted.

You use any two points on a DEAD STRAIGHT part of the curve,obviously the greater the interval between then the more accurate will the calculation be.In the early part of the test internal stresses or even buckles are likely to be present.Internal stress is ,I think,the cause of a phenomenon known as Bauschingerism.

by hooke's law with in elastic limit stress is directly proportional to strain. The ratio between linear stress to linear strain with in elastic limit gives a constant which is known to be young's modulus. so you can use all the points to calculate young's modulus as long as they are with in elastic limit.

All of them, as long as they're experimentally valid points.