Most Slingboxes have always had at least 3 video inputs, the problem is with the audio. Normally the Composite (Yellow), Component (Orange/Green/Blue) and S-Video inputs share a single Audio (Red/White) input. The Pro-HD is the only box I know of that has 2 different audio inputs (although I think the 500 has an SPDIF digital audio connection). If you are using the HDMI port on the Slingbox 500 without linking to the component then the HDMI sound is used.
However, you can get around the problem by using 'Y' adaptors on the Audio leads so 2 or more boxes feed into a single audio input. Then you just make sure all the devices you are NOT viewing ar switched off. Personally I have 3 devices feeding into my Slingbox Solo by using that method.