Three years is a long time. There is generally no regulation of management companies like there is for talent/literary agencies, so I suppose they can write in whatever they want, but I'd be hesitant to sign with a rep for that long.
For WGA signatory reps, Rider W of the WGA basic agreement states "The term for a services representation agreement, whether initial or otherwise, shall not exceed two years."
Also, if you submit a piece of material to the rep and they do not sell it with a year, you can notify them that they no longer rep that material for sale and are not entitled to collect commissions on it.
There are also clauses stating that if the rep doesn't procure you work equaling $10,000 in the past ninety (90) days you can terminate the contract.
You can read the whole thing here: http://www.wga.org/uploadedFiles/wri...cts/agency.pdf
If there are no protections for you in this three year contract, I'd be wary. Good luck!