12-3-30 is a treadmill workout: 12% incline, 3 mph, 30 minutes. Popularised by TikToker Lauren Giraldo. Not concerning, generally safe exercise. Worth being aware if your teen is dieting heavily alongside it. If your kid uses it constantly, ask them where they first heard it, not to police their speech but to understand which online community they are in. Most slang of this kind moves through TikTok and group chats, so seeing it in your child's vocabulary just means they are plugged into normal teen internet culture.