In order to use a computer getting its internet service via wifi as a wifi hotspot, you need 2 wifi adapters in the computer - one to talk to your router, the other to talk to your tablet and laptop. (Technically it's not connecting to a router, it's connecting to an access point - which are in the same box in home equipment, which is why we say "connect to the router". In TMobile's case, they're probably different pieces of equipment, but electrically it's the same thing.)
However, since ST doesn't allow tethering (that's what using a hotspot is called - you tether the laptop to the phone's wifi connection), you'd have to be close enough to a wifi router for the S3 to pick up the wifi signal. That would mean that your laptop and tablet are close enough to pick it up too, so you don't need a tether. (As far as I understand it, the thing that connects to TMobile's wifi is the login - and you can use the login on any wifi-enabled computer, whether it's a phone, tablet or laptop.)
As for what belodion was talking about, either ST or TMobile will cancel your account if they catch you tethering and it's a violation of your contract. In the case of ST, all you lose is the balance of what you've paid so far (the rest of the current card) and your phone number. In TMobile's case, if you're still on contract, you also pay the early termination fee. They canceled, but you breached the contract, so in effect you caused the early termination.