I have an older version of the Slingbox Pro/HD (with the HD dongle), but I think the current version is the same with 3 inputs (one HD and two non-HD). I honestly haven't seen much difference between the two even over local WiFi. You can even do an S-Video in on the Pro (the one I have) so you could control 3 of the boxes.
There is also a cheaper SlingBox Solo I think that just does one input, but that is fine from a quality perspective. I use the Component and Composite outputs from my two TiVo's into the Slingbox and it works great. Downside is no IR 3.5mm jack, so it uses IR blasters to control the two boxes. The Sling can handle remote codes up to 4 I think, so you should be good with codes 2 and 3. No issues having two on the network, they don't stream unless they are connected to.
Enjoy!