The best way to approach plotting a filled polygonal region is to use the "patch" function,
which requires the (x,y,z) coordinates of each vertex of the shape.
For a triangle, you have three equations, each corresponding to one side, so your three vertices will be given by the intersection points between different pairs of lines. To calculate intersection points, use the "solve" function. For an example of how to use "solve" on a system of two equations, type the following into the MATLAB Command Window:
>> openExample('symbolic/SolveMultivariateEquationsAndAssignOutputsToVariablesExample')
Also, once you have plotted the 3 vertices with "patch", use the command "view(3)" to see the plot in three dimensions.
