ESPN3 is accessible based on your Internet Service Provider. Here's a full list of those:
http://espn.go.com/espn3/affList
If you use one of those providers, then you will be able to watch this week's game against Troy either through your computer or on your XBox or whatever other apparati can do ESPN3.
WatchESPN is accessible based on your Television Provider and television package. Here's a full list of those: Time Warner, Verizon FiOS TV, Bright House, Midcontinent, and Xfinity/Comcast. It's also the only one of the two that offers an iOS app.
ESPN changed their policy regarding ESPN3 this year. Previously, the only games that were blacked out on ESPN3 were the FSN SEC games within the SEC footprint.
This year, games on ESPN, ESPN2, ESPNU, the weekly FSN SEC game (and maybe the 12:21 SEC Network game) will be blacked out on ESPN3 in areas of the country where that's the game being shown on a live ESPN channel. For example, since interest in Ole Miss vs Central Arkansas was so small, the areas where that game was actually carried live on a primary ESPN network were few and far between. This gave more of the country access to the game. Conversely, Clemson-Auburn was blacked out almost everywhere. Count on your Tuesday, Thursday, and Friday games being blacked out on ESPN3 each week too.
Also, this is the first year they're going to have exclusive ESPN3 games, which means the ONLY way to watch it is by having ESPN3 (or buying it through Gameplan). For those of us in Texas, this has been the norm for us for a long time.
Last week's game vs Jackson State was blacked out in the SEC footprint because it was the FSN game.
If you want to watch the game being played on ESPN, ESPN2, or ESPNU somewhere other than a TV with access to ESPN, ESPN2, or ESPNU, you'll need a television provider that offers WatchESPN or use a service like playon.tv and get the FirstRowSports.eu plugin.
Hope that helps.