A critical missing bit of information is missing from your questions. An error is thrown by the last line in your quesiton.
Error using plot3
Not enough input arguments.
This is because your plot3() inputs are cell arrays. It looks like your loops produce scalar values on each iteration so those values can be stored within a numeric array which will be much easier to use.
Your loops were overwriting the x,y,z values on each iteration of the j and k loops. The method below will store those values but it's up to you to make sure this makes sense in the context of whatever you're doing.
X = nan(length(L), length(rho), length(theta)) ;
Y = X;
Z = X;
X(i,j,k) = x ; Y(i,j,k) = y ; Z(i,j,k) = z ;
As for plotting, it's not clear how you would like to use these values. plot3() creates a 2D plot but the inputs cannot have more than 2 dimensions and the x y z values above have 3 dimensions.