How to fix a three strand twisted polyester string

These type of strands usually curl/braid around each other.

1) Start by twisting each individual strand in the direction it's leaning towards (this is often clockwise but might not be.)

2) Next, group them together and wrap them tightly around each other in the opposite direction of whatever you just did. Do it tight enough that they wrap around each other.

3) Make sure you're holding tension during all these steps so that the twists stay together.

4) Tie a good knot at the end so it doesn't unravel again.

If this really isn't working, you can take the string out and replace it with a shoelace or another string. Shoe laces are pretty easy to thread through pants due to the aglet at the ends that keep them from coming undone, but any appropriately-sized strand will work. A quick tip you can do to make this faster is to use a safety pin to attach the new strand to the old one temporarily. Pull the old strand out which will pull the new one in. Once the new one is in, you can unpin it from the old one and throw the old one away or reuse it for whatever project you want to.