YOu may follow something like this:
[X,Y,Z] = peaks(50) ;
% Pick a point
[val,idx] = max(Z(:)) ;
P = [X(idx) Y(idx) Z(idx)]' ;
figure
hold on
% waterfall(X,Y,Z)
plot3(X,Y,Z,'.b')
plot3(P(1),P(2),P(3),'*r')
L1 = [P [X(idx) Y(idx) 0]'] ;
L2 = [P [X(idx) 0 Z(idx)]'] ;
L3 = [P [0 Y(idx) Z(idx)]'] ;
plot3(L1(1,:),L1(2,:),L1(3,:),'r')
plot3(L2(1,:),L2(2,:),L3(3,:),'r')
plot3(L3(1,:),L3(2,:),L3(3,:),'r')
