Actually, HS is more connected with the other two than most people realize. While it takes place on earth instead of in space, HS is the climax of the entire series. Without HS, the series would not communicate the real point of the series.
While my favorite is definately Perelandra, HS is a MUST read. If you are having trouble with it, or getting bogged down with it, I suggest reading The Abolition of Man and then trying HS again. Lewis himself said that HS is the fiction version of AoM.
I agree that HS does not seem to "get to the point" as well as OSP or HS, but I think that is because the "point" is harder to get to. Lewis was dealing with difficult concepts and putting them into words is very difficult. That he was able to do it at all was amazing, and yet he not only put his ideas into words, he did it in a beautiful and captivating way.
All this to say, Read the first two, read HS, then read Abolition of man, then read HS again. You will be amazed at how much more you gain in the second reading (of course, that goes for all good books - hence C.S. Lewis's quote below).
Oh, and just for the record, I, too, LOVE HS