Let me clarify, how subplot function works. Subplot(m,n,p) divides the current figure into mXn grids and creates axes in the position specified by p. For example:
subplot(3,2,3)
It will divide the current figure into 3X2 grid and plot the specified image or plot to the 3rd position or grid 2,1. MATLAB numbers position by row.
So, if you want to plot three images you can divide the figure in 1X3 grids or 3X1 grids. Code for 1X3 grid is given below.
subplot(1,3,1)
imshow(img1)
subplot(1,3,2)
imshow(img2)
subplot(1,3,3)
imshow(img3)
For more information, you can check the documentation.