Either burst mode or AEB mode is engaged. Could also be J+R mode, meaning one picture is a JPEG and another is a RAW/DNG, and then you took 3 pictures. Or possibly some combination of those things.
For reference, these are commonly used by professional photographers. AEB allows you to blend the photo using the best exposure from each photo (oversimplified explanation), burst shots have their advantages as well. Finally, J+R is great when you want to have a quickly usable compressed image to send say to a client, but the RAW is the one you do your post editing in Lightroom or Photoshop etc to bring out the best lighting and colour (again, oversimplified explanation).