use just one port on the lnb... Use the #1 port straight to your receiver.
You don't have three lnb's, There are three feedhorns on the lnb, not three lnb's.
CAUTION:
Some RV's have a splitter in line. If you have more than one wall plate in your RV and only one cable outlet outside your RV then chances are there is a splitter in line somewhere. If there is a splitter in line then your receiver will have check switch problems. Either bi pass the splitter or remove it completely. It must be a straight shot from the LNB/Dish to your receiver.