Your current 3-LNB dish provides 4 outputs that each can access signals from all 3 satellites. To preserve access to all 3 satellites, you need to add a cascadable 4x8 or 5x8 multiswitch, connect all 4 outputs from the satellite dish to it, then run wires from 5 of the 8 outputs of the multiswitch to each of the tuners on your receivers.
If you don't care about programming delivered via the 110 and 119 degree satellites, you can get by using a 2xN or 3xN multiswitch connect to two of the 4 outputs of your dish, then connect your current receiver and the new one to the outputs of that multiswitch. Anything connected to that multiswitch will only get programming from the 101 degree satellite. You can leave your Hughes TiVo DVR connected directly to the satellite dish, and it will still receive programming from all 3 satellites. Or you could connect a 2x8 or 3x8 multiswitch to two of the 3 current lines from the dish that you have, then connect both TiVo DVR's to the 2x8 or 3x8 multiswitch. Maybe that's more convenient. But again, only programming from 101 degrees will be available to the receivers connected to it.
The first option (the 4x8 or 5x8 multiswitch) is the most flexible, and leaves you able to add more receivers, including HD receivers, but does mean you need to run two more wires from your dish (or wherever you locate the multiswitch, which itself needs all 4 wires from the dish conected to its inputs).
It does not matter which outputs from the dish you conect to a 2xN or 3xN multiswitch; the dish will send the signals the multiswitch needs.