Column density integrated over Z-direction and Y-direction (left and middle col., respectively) and midplane density (right column) for three different snapshot times from top to bottom for run 3S1-hr. The projections of the three massive stars into the X–Y plane is indicated as small red stars in the density plots on the right. The 60 M_⊙ star blows the biggest bubble from the origin. The 32 M_⊙ bubble towards its lower left (XY-plots) is only slightly bigger than the one of the 25 M_⊙ star above. The shell forms spikes and dense clumps due to the combined action of Vishniac and thermal instability. A movie is provided with the online version.