Apologies for the late reply.
We have thought about 3 lines, but are concerned that 3 would simplify rather than make the puzzle more difficult. We do have thoughts on ways to make it more difficult beyond the 3 lines, but are still unsure as to which way to focus. Any suggestions you might have on how to make the game difficult are welcome.
We do have plans to implement a way to show when a tile is completed and when a tile is not. We are also planning on implementing a zoom and scroll aspect in the Android version, to allow for larger games.
Thanks!