Fit values for a CFA with a single factor can only be computed with models with more than 3 observed variables. With 3 obeserved variables and one factor you have to compute 6 free parameters (with one fixed). But your covariance matrix has also 6 nonredundant elements. Hence you get df=0 degress of freedom. A chi-squared distribution with df=0 is constant 0, hence we can expect the test statistic to be 0.
