Speech processing is an important element of affective computing. Most research in this direction has focused on classifying emotions into a small number of categories. However, numerical representations of emotions in a multi-dimensional space can be more appropriate to reflect the gradient nature of emotion expressions, and can be more convenient in the sense of dealing with a small set of emotion primitives. This paper presents three approaches (robust regression, support vector regression, and locally linear reconstruction) for emotion primitives estimation in 3D space (valence/activation/dominance), and two approaches (average fusion and locally weighted fusion) to fuse the three elementary estimators for better overall recognition accuracy. The three elementary estimators are diverse and complementary because they cover both linear and nonlinear models, and both global and local models. These five approaches are compared with the state-of-the-art estimator on the same spontaneously elicited emotion dataset. Our results show that all of our three elementary estimators are suitable for speech emotion estimation. Moreover, it is possible to boost the estimation performance by fusing them properly since they appear to leverage complementary speech features.