I have troubling the understand linear separability issue. As far as I search on the web,
To be linearly separable, you have to able to separate data with a straight line for 2D, plane for 3D,
From that sentence, I understand that, I have to separate data with a single line if it is linearly separable. If I'm right, I wonder how could I separate my data into 3 different classes with a single line? It seems impossible?
It's just a definition. If you can separate classes with a straight line (in 2 dimensions), the data are linearly separable.
If you have 3 classes in 2 dimensions, they are linearly separable if you can separate each class from all others singly, like this
a | b | c
or something similar. (This is sometimes called the "one versus all" strategy.)