[Shostak]

If you want to limit travel to the third star, stay away from jump drive and stick with constant 1G acceleration--this model is a great way to keep things theoretically plausible and it provides hard-sci artificial gravity. Using this system makes acceleration and fuel storage/consumption huge factors in any travel.

To keep the third star inaccessible, make refueling difficult. This also keeps things believable; after all, why should refueling be an easy option, especially in the emptiness of interstellar space? Since constant acceleration at 1G would eat up a lot of fuel, just declare that it is an in-system-only option--Maybe just a few AU (an already enormous distance). And make refueling take a long time, requiring lengthy layovers--especially long ones if there is not a fuel depot established. This is related to Jason's suggestion that the fuel would only be enough for 1-way, since there would be no industrial infrastructure to support refueling at the destination. Travel to and fro would require an investment of human resources and decades of construction at the far end.

You could also make the orientation of the planes of the systems to each other work to your refueling advantage. Since most baryonic matter will be found roughly within the disk of the the ecliptic, if the stars had roughly parallel ecliptic planes most refueling stations (presumably servicing spacecraft utilizing the raw materials of asteroids and planets) would likely be no closer to the other system than the stars themselves. So there is no convenient half-way point. As a bonus, this coincidental arrangement provides a fertile field for all kinds of "Intelligent Design" theory subplots.