A "week 3" bud hardener is a reference to products containing the plant growth hormone (PGR) pactlobutrazol. "Gravity" contained paclo, but I'm no longer sure that it does as the formula was changed some years back (smells weird now also).
I highly recommend against paclo as it is detrimental to the health of the cultivator, smoker, and more importantly, Mother Earth.
Besides that, it negatively effects terpene production, trichome production, and the complexity of the nose/high/stone of your herb. You can grow way better buds without it. You can even achieve greater yields without it (as it decreases "oil" weight). The only thing it does is make up for a lack of skill/effort/passion in the final weight. I call it "cheat code", but the final product is anything but "victorious".