the hr20 works with any directv satellite, when u first fire it up, itll ask you to choose a satellite setting. the only con of using a 3lnb is that you are not going to get ur locals in hd when the launch in your area. if thats what ur anticipating, call dtv and get a kaku upgrade (5lnb), youre going to need it for future upgrades with hd. the good news with the 3lnb is that ull still receive the basic hd channels (70-79).
My HR20 is hooked to a 3lnb phase III, since we don't have HD locals yet. I would highly recommend having 2 lines from the dish to the HR20. This will make the setups/downloads/recordings much easier.
Sure it will work with the 3LNB but it won't get any of the new HD channels going up nor your HD locals (if/when available).
No reason not to get the 5LNB dish, they will install it for free. You will need it for the future.