It’s just a class. Here’s how you would use it, replacing all the lines you wrote above.
var rq:RoundedQuad = new RoundedQuad(100, size, size, color);
It looks like you use a different colour later, but that’s easy to do
rq.iColour = 0xFF00FF;
The biggest benefit is that if you have three then that‘s three Textures so three draw calls. If you use untextured Meshes they can all be drawn with one draw call. One or three draw calls is no big deal, but it adds up if you have other objects that are drawn similarly.
Though if you still use BitmapData and Textures you can reduce the number of Textures and draw calls. Create one large BitmapData, large enough to contain all of your rounded rectangles. Draw three rounded rects into this, offset from each other so they don’t overlap. Make note of their bounding rectangles.
Then upload this to a Texture and create SubTextures for each rounded rect, using the saved bounding rectangles in the constructor of each one. You save draw calls and Texture uploads. You can add other bits of art to the same Texture, creating your own texture atlas. It seems complicated but all you need to keep track of is the bounding rectangle of each.