If each plant has three of these bugs how many are finding mates?
The insects in your images have several different common names, among them Hedge-nettle Stink Bug and Happy Bespectacled Stink Bug. (The Latin name is Cosmopepla conspicillaris.) The paired set are a male and female.
Little information is available about them. Thus, I'm unable to suggest how many might result from mating.
During my research, I located one sourcewhere they are listed as potential plant pests. [On page102 in the book "Insects of the Pacific Northwest"] Even so, we seldom receive questions or complaints about them.
See "Stink Bugs of Oregon" for brief text plus images of their eggs and younger stages - http://www.oregon.gov/ODA/shared/Documents/Publications/IPPM/StinkBugGuide.pdf.
If you have only a few, flick them into soapy water where they will drown.
Thanks, Jean! There must be 300 or more and as long as they're not a destructive beetle but a stink bug, I have no worries.