The Canon EOS 1100D (Rebel T3) is an affordable entry level DSLR which does not provide a dedicated button for the depth of field preview near the lens like the more expensive models, as the 550D. But in fact this function is available and can be configured to be activated with the “SET” button.
For this you select in the setting “Custom Functions (C.FN)” in the menu, then go to position 8 for “C.Fn IV:Operation/Others • Assign SET button” and there you can change the function of the button from “0:Normal (disabled)” to “5:Depth-of-field preview”.
Now you can press and hold the SET button in recording mode to close the lens aperture and evaluate the depth of field. This works with the optical viewfinder and also when using the preview (LiveView) on the display of the camera.