The third stem (after cut to the right length) is placed carefully into the vase, resting lightly on top of the point where the first two flower stems cross. If you use roses, it is wise to remove the thorns before designing. Not only will that save the bride's fingers, but it also helps the flowers slide into place without catching on the thorns.
These three stems are all the same length and resting against the top lip of the vase.
Here's the top view. It is eventually going to get a lot more crowded in this vase, but the stems of the flowers will begin to form a natural grid that will help "interlock" those flower stems together, maintaining a natural flare outward.