Well topping a plant with 3 weeks left won't accomplish much else than giving you a premature main koila to smoke, and a big hole at the top of the plant

There is a way to accomplish what it is you are asking but it is a very very very fine line you dont want to make a mistake on.

Removal of fan leaves will stop much of the plant from growing and will force it to focus on flower growth and preservation of the flower.

If you read the grow thread on Whore from this summer, I did just that. But I may have done it just a slight bit too much, but temps in 90's forced me to.

Removal of fan leaves and half of the fan leaves (cut them in half individually) are two ways you can attempt this. But I swear, one leaf too many can force the plant into an early harvest.

Done successfully, it will help final swelling of the flowers.

Good luck.