We aren't told anything about the orientations of ABC, so there are three cases to consider:

1) The points line up ABC: Then d(AC) = d(AB) + d(BC) = 3 + 2 = 5.

2) The points line up ACB: Then d(AC) = d(AB) - d(BC) = 3 - 2 = 1.

3) Any other combination where the points are not co-linear will have distances between 1 and 5.

So is the best that we can do.

-Dan