When I kept Multis, I had one male to three female and they bred like crazy. I had a similar set up to yours. My male was about an inch and a half and my females were three quarters of an inch or so. I read that there really isn't any aggression between males. I read one article that stated that two males had moved their shells right next to each other without any aggression issues. It looks like you have a lot shells in the tank for only three fish. Maybe you should thin the shells out a bit and you might have more luck? Other than that, I think adding more females should do the trick. Though, that doesn't help the problem with finding females in your area. I'm in the states and I've got 3-4 great lfs and they rarely get multis in.
What is your water like? The water from my tap is very hard and has a high ph which might account for the ease in which my multis bred.
Good luck. They are an awesome fish to keep even if they don't breed. I loved watching them excavate the tank - bury this shell, move that shell. It was a lot of fun. Don't give up yet.