Lets look at sulfur trioxide. Here you have the sulfur in the middle surrounded by 3 oxygens. In order for 3 oxygens to be evenly spaced, they form a triangle (this is different than the cube formed when each carbon was connected to 4 atoms, because here there are only 3 other atoms). The actual shape is called trigonal planar, which means that all the atoms are in one plane (in 2D, not 3D like the carbon). Here is a picture of sulfur trioxide:

Here, the middle yellow atom is the sulfur and the three red atoms are the oxygens. You can see they are evenly spaced, in a triangle arrangement. You can see from the periodic table of elements that each oxygen has 6 valence electrons and wants to share 8-6 = 2 electrons, which is why each oxygen is connected to the sulfur by a double bond. Sulfur is a little bit special- here, sulfur is sharing 2x3 = 6 electrons with the oxygens. This has to do with sulfurs electron arrangement. You can figure this out from the periodic table of elements, but to clear up all of the rules of electron sharing, you should ask your chemistry teacher or look at your chemistry book.