If you've been using HCG during the cycle then 3 weeks post cycle is probably over kill. When used for PCT, HCG is best used the last 2-3 weeks on cycle for 2 reasons:
1.) HCG itself is suppressive of the hypothalmus (and pituitary) so it's use post cycle will delay recovery
2.) HCG will restore testicular function (and raise testosterone levels) even with gear still in your system.
For example, if you were using test enanthate on your cycle, you wait 2 weeks after the last enanthate shot to start clomid anyway. Using HCG during this time (the 2 idle weeks) is perfect; it will make sure the testes are ready for recovery and it will keep your testosterone levels high even as the enanthate is working out of your system.
Then clomid and/or nolvadex is used for 3 weeks to hasten the recovery of the hypothalmus and pituitary.