A new signal synthesis and tracking method is developed for a 3-Lead wireless electrocardiography (ECG) system. Exploiting the temporal correlations in ECG signals, the standard 12-lead ECG signals can be synthesized with high precision by using only three differential pairs of electrodes. The correlations between the original and synthesized ECG leads can be as high as 0.98. And the separation between the two electrodes of a pair can be drastically reduced to around 10 cm, which is about the size of a medium adhesive tape. Experiment results also show that the proposed ECG synthesis method is much more robust to variations in acquisition positions when taking into account the spatial correlations among the ECG leads in syntheses. This greatly improves the feasibility of using fully wireless ECG systems on the long-term monitoring and diagnosis of heart diseases.