The Tamron may need an autofocus microadjustment to work well with your specific camera. Unfortunately, the T3i doesn't have that feature.

You might try borrowing a friend's 50D, 70D or 7D to see if that's the case. But really, it would simply be an academic exercise. The lens would need to be returned regardless of whether or not it was an AFMA issue or a lens performance issue because of the body you're using it with.