With 3 subs you have three alternative configurations depending on what you are trying to achieve. Most people would connect all three subs to the receiver sub out and place them at sweet spots throughout the room to flatten nodes and achieve more linear FR. When attempting this I would pay particular attention not only to placement but also to balancing gain, phase and delay with each of the subs and the main speakers so they don't counteract one another.
Some people believe connecting one dedicated LFE sub like you have done to the DVD players sub out reduces distortion and other artifacts at the crossover point. This connection does not redirect bass from the other speakers. Again care must be taken with phase and delay issues, this time with the other subs.
That leaves two identical subs, if I read you right 2 500s, which I would suggest you configure as stereo subs. That means running a full range signal to the right and left 500s as well as having the flexibility to cross the speakers to the 500s. Because there are no high level outputs on the 500s owing to their digital design the individual L/R crossovers need to be done at line level to each sub which your receiver/processor is likely not capable of doing. To do this you would need an external crossover unit such as can be found in an SMS-1 equalizer. The results can be outstanding, if done properly.
If the stereo sub alternative is too costly or you don't have the gear then I would suggest dedicating one sub to the LFE channel plus surrounds and setting up each mono 500 by the main channels and crossing as low as you can, ideally below 80hz for best results. As above take care about phase, delay and blending with the mains at the crossover point.
