The label "208V 3 phase" would imply that there is no neutral conductor present., and the neutral blade on the 14-30 receptacle is being used for one of the hot phases. If the system had a neutral, it would be designated as "208Y/120V".
You need to have an electrician open up the outlet box, and see if there is a neutral conductor in the box. If there isn't, you will need to install one to use the L21-20 receptacles. I would advise having this done ASAP, as adding a conductor will be much easier during construction than after the sheetrock is up.
