pandapower uses Sequence Frame to solve three phase power flow :
pandapower.pf.runpp_3ph.runpp_3ph
Note
If you are interested in the pypower casefile that pandapower is using for power flow internally, you can find it in net["_ppc0"],net["_ppc1"], net["_ppc2"]. However all necessary informations are written into the pandpower format net, so the pandapower user should not usually have to deal with pypower.