Do I need to pay duties/ taxes when I purchase clothes from China to the US over 3 times?

Most articles of clothing from China are dutiable, however the duties on most of them are pretty low. USC&BP does not collect duties where the cost of collection is less than the duty collected, around $25 or so. If you are only ordering a few at a time you probably won't pay any duties. If you order commercial size shipments -- typically several hundred items -- you will be charged the duties on the goods.

The risk with ordering goods from China is that there is a LOT of counterfeiting going on. If counterfeit goods are discovered by USC&BP they will be confiscated and destroyed without compensation. That leaves you to try to recover from a foreign vendor. Your credit card company will not reverse the charge if the purchase was illegal which pretty much leaves you swinging in the wind. Stay away from brand-name goods and you should be OK.

You could have paid duties the very first time you imported something. I have.

If this is coming through the post office, they may or may not be tracking it. If they are tracking it, they may be going by number of shipments or total declared value.