Demux from 1 camera to 3 DVR's
What you need is a "distribution amp" (or DA) which is appropriate for the type of video signal. For example, you could search for a "composite video distribution amp". Make sure you get one with enough outputs to go to all of your DVRs.

Also, check to see if you can remove the termination on your DVR inputs (for example, look for a "High-Z" switch). If so, disable termination on all be the last and daisy-chain using tee connectors.