If you do not need the "wave" contents for a further use, just drop out (k) from all its entries.
If you need it for a further use, initialize it as
before the loop and address it as wave(k,:) within the loop.
Commands "stem" are definitely incorrect. If you want to see plots of the dependences on t, they should look something like stem(t,combined) or plot(t,combined), etc. However, the t range in the second version (0,3) is much wider than the one of the first version (0,0.01): consequently, such a graph will show much (hugely) more oscillations--you will probably see nothing but a unifromly painted square.
I do not see any need in the "nold on" and "hold off" commands in the both version.