When you get an error in matlab instantly set the debugger on:
then rerun the command that caused the error, this will give you an interactive command-line prompt at the line where the error happened in the function, in the stack of function-calls. This makes it possible to inspect the different variables in the function-workspace, and move up in the call-stack so that you can inspect the input arguments and such.
In this case you problem is that when ode15s calls the function mo it starts off with your initial conditions, converted into a column array - that is a 3 x 1 array. In the function you first calculates dy which will be a 3 x 1 array, then dv which will be a 3 x 1 array and then dw which also will be a 3 x 1 array, then these 3 arrays are concatenated to produce a 9 x 1 array.