If you have 1 HDTV and 2 SDTVs, the cheapest solution is to get one Multi-room DVR (same box as a regular DVR, and all of the DVR's are HD) and two SD STBs. The SD STBs will be able to access recordings on your MR DVR.
The shortcomings sof this setup are:
(1) you can only setup recordings from the MR DVR. You cannot do so from your SD STBs;
(2) you must pay attention to what you are recording. If you record an HD channel, you will NOT be able to watch it on your SD STBs (it will not downconvert!);
(3) If you have folks watching 3 TV's, it's VERY possible that you will not have enough recording space on your DVR. If you stick to recording SD programming, you might be OK, but the HD programming eats up that 160GB Hard Drive pretty quickly (~20 hours worth)
I have 3TV's as well, but 2 are HD, and I opted for 2 DVRs and 1 SD STB simply because of the paltry amount of recording space. So the SD STB can access one of the DVRs (set up as MR DVR), and the other DVR is a standalone unit.