Is this circuit correct?
Simplest form of load balancing is to add a series resistor to each diode. But you lose some voltage. 

You were losing some voltage anyway, because of the diode. 

It works as follows. The DC/DC converter with the highest output voltage and the lowest diode drop will start supplying current first and the most. But, with increasing current, the voltage over the series resistor causes the output voltage to drop. That allows lower voltage supplies to catch up and the system more or less balances out. The voltage drop over the diodes, which also varies with current, will help a bit with balancing

As I said, that is is simplest method. A much more complex but efficient scheme would be if you measure the current from each output and use that to "somehow?" control the output voltage. Whether that is possible greatly depends on what the DC/DC converters offer for a feedback mechanism. 

If the DC/DC converters have a constant current mode, you could just wait for that to kick in and distribute the load. (It will not be balanced.)  The disadvantage is that the converter(s) with the highest output voltage will be used most of the time and the others will follow one-by-one. Again the voltage drop over the diodes will help a bit with balancing.