I would also normally think safety shift is an issue but a quick perusal of the Rebel T3 manual doesn't show this option under configuration options so something else may be going on with this camera unless the option simply isn't listed in the manual. (and on edit, I see that kvbarkley confirmed this).
I know that resetting the camera is the go to option but in many cases I really think that it is better for the user to dive deep into the manual to understand what change was made because the more you understand what the camera system can provide the better you can utilize what Canon designed into it. And over time many of us use a lot of custom options to personalize the camera and resetting everything back to zero requires recreating the personalization. On the 1DX series bodies there are a lot of options and I try to keep a running history of the setup and why in an Excel spreadsheet but I am definitely not perfect with keeping it up to date and I wouldn't want to go back to ground zero and start over again.
To the original poster, please let the forum know what you find because someone else will likely run into this same issue in the future. Although not likely, it could be the encoder control used for selecting shutter speed has gotten dirty or is malfunctioning. These encoders, especially if mechanical instead of optical, can be problematic with age in many devices producing their BCD output when none is desired/commanded. Try tapping but not actually rotating the encoder control to see if this causes a speed change.
Rodger
EOS 1DX M3, 1DX M2, 1DX, 5DS R, M6 Mark II, 1D M2, EOS 650 (film), many lenses, XF400 video
Anytime things don't go as expected, reset your camera. It is often easier than trying to find the misplaced setting.
