If you already have an AT&T contract, I'm not sure an unlocked phone will even allow you to get around paying for tethering. Even if it did, you'd be paying $449+ for the phone, versus $99 on contract, which pretty much negates the savings of going with the 3GB plan over the 5GB plan with tethering.
I wasn't clear on whether or not you already have an AT&T contract, but if you don't and you're looking for some lower cost options, here are the two I would consider:
Buy a Lumia 710 from T-Mobile ($199 after $150 rebate) and go with one of their no contract Monthly 4G plans. You get unlimited talk, unlimited texts and unlimited data for only $60-70 a month. For $60 you get 2GB at 4G speeds or for $70 you get 5GB at 4G speeds (once you go over, they throttle your data speeds, but it's unlimited data) You will have to wait for the Tango update until you can use internet sharing on the Lumia 710 however, but that update is coming VERY soon, way before Apollo. The Lumia 900 will NOT work with 3G/4G speeds on T-Mobiles network currently, so an unlocked one would only get you 2G speeds. If you can wait until summer, T-Mobile is supposed to get the Lumia 900 then.
Buy a Lumia 900 for full price ($450 or so) and go with a Straight Talk no-contract plan. They use AT&T's network, so the Lumia 900 will work at 3G speeds for you today. You do NOT get access to 4G/LTE with Straight Talk however and there are some limitations to the "unlimited data" plans. Supposedly they don't permit streaming video and I've heard of people getting cut off for "excessive data use" and they do not permit tethering which could be a deal-breaker for you (though there are allegedly some workarounds for this) The good news? It's very,very cheap. $45-60 a month for unlimited everything.
Personally if I were on a tighter phone budget, I'd go with the Lumia 710 on T-mobile with the $60 a month Monthly 4G plan. With the rebate, you get a great phone for only $199 and no contract. You should able to tether in a few months when the Tango update comes out and 2 GB at (near) 4G speeds is enough for most people.
I don't know of any cheaper way to get what you're looking for than that. Hopefully that is helpful to you.