The question here is how many coax connectors do you have on your LNB? if you have two, then you can use a multiswitch to connect the two standard and one DVR receivers to the dish. If, however, the LNB has only one coax connector, then you need to replace the LNB. It should be easy to find and replace. Most RadioShacks carry it, you might find it at WalMart, BB or CC, and if you shop online, there are many places to buy, including ValueElectronics, a supporter of this site.
BTW, when someone refers to a "single" or "dual" LNB, they are referring to the number of outputs on a single LNB. If they are referring to a dish capable of receiving multiple satellites, they will talk of a 2, 3 or 5 LNB dish.