While you cannot grow new bone in 90 days, you can maximize your existing stature and potentially "gain" 1–3 inches by addressing spinal compression and posture.

During the peak of puberty, males may grow about 4.4 inches per year , while females average 2.5 to 3.5 inches annually . Gaining 6 inches in only three months far exceeds even the most rapid natural growth spurts.

No, it is to grow 6 inches taller in just 90 days once you have reached adulthood or if your growth plates have fused . For most people, height is 80–90% determined by genetics.