I'm not a big garlic treatment fan, but I may be the exception. You can probably put a lot more than 3 hippos in your tank, especially if you add them small. However, keep this in mind. Mortality among small tangs can be high because of a myriad of factors, including the fact that tiny ones often get sucked into overflows or powerheads, and they are susceptible to ich. I think they pretty much will get it when you put them in, so if you're starting out right, there are posts about the lifecycle of ich being around 30 days or so, so I would quarantine the baby tangs first and treat them with something that will kill ich, instead of garlic, which will merely help them shed it through the slime coat being stimulated. When they've been treated, toss them in the big tank. Or, you could just let nature run its course and see if your tangs get ich. If they do, and your tank is healthy, they'll kick it without any help from you and be fine.