  A coupled dark energy model is considered, in which dark energy is
represented by a generalized three-form field and dark matter by dust. By
assuming the functions $N$ and $I$ in the model's Lagrangian as two power-law
functions of the three-form field, we obtain two fixed points of the autonomous
system of evolution equations, consisting of a attractor and a tracking saddle
point which can be used to alleviate the coincidence problem. After
marginalizing the present three-form field $\kappa X_{0}$ which is unable to be
strictly restricted, we confront the model with the latest Type Ia Supernova
(SN \uppercase\expandafter{\romannumeral1}a), Baryon Acoustic Oscillations
(BAO) and Cosmic Microwave Backround (CMB) radiation observations with the
fitting results $\Omega_{m0}= 0.280_{-0.048}^{+0.048}$ and
$\lambda=0.011_{-0.032}^{+0.032}$ in the $2\sigma$ confidence level, we also
find that the best fitting effective dark energy equation of state (EOS)
crosses $ -1$ at redshift around 0.2.
