@David64 I have the same problem. My house has a queen and two twin beds and my maximum is 4 guests. I don't want people sleeping on the couch either. On rare occasions when I have all men or all women guests and two of them don't want to double up on the queen bed, I have an airbed to accomodate them at no extra charge. Perhaps you should amend your listing description to be sure to get accross the message that the your max is 3 people. I put it in my house rules which I require guests to confirm that they have read and agree to before I accept a reservation request.
Question: Is your nightly price per person or the price for up to 3 people in your home? Perhaps you are getting so many requests because it is unclear.
As for declining affecting your listing status, some hosts say yes, others no. It hasn't affected mine and I have acheived Super Host status. If I am not comfortable with a request, I decline it without regards to possible consequences. My priority is my peace of mind.
Hope this helps a bit!