How to add solar DC output, Wind DC output and Fuel cell DC output to create a DC bus
Why "considering the DC voltage is the same for all." The voltage won't be the same for all unless each one includes a DC to DC converter that will assure that. What you are asking is "how do I go about designing a DC microgrid?" The basic requirement would be to install DC to DC converters for each source. The converters must be suitable for DC microgrid connection. You also need to consider the connection of your storage batteries and your system for controlling the use of sources. Presumably, you wouldn't bring the fuel cell on line unless the other sources were fully utilized and the storage is nearing depletion.

I think that only a very general overview can be provided here.