Rare triple transits.
Shots taken when moons are briefly moving in front of another usually much more sizeable body, in this case Jupiter.
The round black fuzzy spots on the cloudy surface of Jupiter are the shadows of the transiting moons.
By NASA, ESA, and the Hubble Heritage Team (STScI/AURA) - http://hubblesite.org/newscenter/archive/releases/2015/05/image/b/ and http://hubblesite.org/newscenter/archive/releases/2015/05/image/c/ (see also http://heritage.stsci.edu/2015/05/index.html), Public Domain, Link
