From a NEMA L6-30 3-prong connector in the lab, it measures 212V btwn two prongs and 122V btwn any of the two prongs to the third prong. Based on this info alone, is it a 3 phase 208V system or a 1 phase 208 system?

The L6-20 receptacle is providing single phase 208 volt service between the two live terminals with the 212 volts, the third terminal is the equipment ground.

This circuit comes from a 120/208 volt 3 phase 4 wire system.