I use the cord lock but you are correct, the tassel will be a problem getting it on.

You could run another thinner cord through the same holes and attach a cord lock to that, there by making the other tassel cord simply ornamental. The second cord might be hidden under that flap.

If you can stand to live without the 3rd pair of tassels, just remove them and pull the cord up under the flap with the cord lock. Probably not preferred, but an option.