A wiring diagram of a 3-phase rectifier is given in my public report KD 340 at figure 5 for star and for delta rectification. A 3-phase rectifier has six diodes. One can use a complete block with six diodes in it, one can use three blocks with two diodes in it or one can use six separate diodes. I assume that you have used the last method. For this method, the three upper diodes can be pressed in one heat sink and the three lower diodes can be pressed in another heat sink. The heat sinks must be electrically isolated from each other.
Two different diode types have to be used for two heat sinks. You have types for which the housing is positive and the terminal negative and types for which the housing is negative and the terminal positive. So in one heat sink you have to use diodes with a positive housing and in the other heat sink you have to use diodes with a negative housing. The terminal of the first diode on the upper heat sink is connected to the terminal of the first diode on the lower heat sink. The terminal of the second diode on the upper heat sink is connected to the terminal of the second diode on the lower heat sink. The terminal of the third diode on the upper heat sink is connected to the terminal of the third diode on the lower heat sink. So to these three points, the three phase wires of the generator are connected. One heat sink becomes the positive terminal and one heat sink becomes the negative terminal of the rectified DC current. It might be that you have used six diodes of the same type and this doesn't work if you have two heat sinks. You can use six identical diodes but in this case you also need six separate heat sinks which are all electrically isolated from each other.
If the wiring of the diodes is correct, the two diodes of one phase are connected in series and the plus of one diode is connected to the min of the other diode. This means that these two diodes guide a current in one direction and no current in the other direction. But this can only be checked for six separate diodes. If you find that the two diodes of one phase guide no current in both directions it means that one or two diodes are blown or that the diodes are connected to each other with the same polarity.
