That's all well and good, and will be a great start for you, however, the T3i only supports full-stop ISO speeds; so Manual with auto ISO could give you a slightly underexposed or over exposed image depending on what the camera thinks is best.
To be clear, when in Av mode, the shutter speed can be adjusted in increments of 1/3; similarly when in Tv mode, the aperture can be adjusted in increments of 1/3... Unless you have a camera that can adjust the ISO in increments of 1/3 also then you might not get the best exposure.
For example... if the correct exposure was achieved with f4, 1/160, and ISO 200 and you dial in f4, 1/200, ISO Auto then the camera will have to decide whether to give you ISO 200 and risk underexposing by 1/3 of a stop because the shutter speed is faster than the best settings, or giving you ISO 400 and risk over exposing by 2/3 of a stop because ISO 400 is more sensitive and the shutter speed becomes too slow... A camera that had intermediate ISO speeds would give you ISO 250 for example.
This isn't a massive issue, however, it's something worth bearing in mind when you're shooting. For example, if it's slightly under or over exposed when you take a shot, you might have to dial in a little more or a little less shutter speed to get it spot on. Once you get more experience you will be able to think about the ISO you require based on the conditions you're shooting in... don't forget there's nothing wrong with having a really high shutter speed for macro work, so you could set ISO 800 for example in Av mode and see what shutter speed it thinks is correct... if it's too slow then try ISO 1600, if it's really fast then try ISO 400 to get a bit better image quality as a result.
Hope that makes sense, Chris