Calculating single phase load from 3 phase power measurements
I don't see any way to do this. You need the power factor of the individual phases and that's not available. The equation is incorrect. 

\$P_{actL1} = P_{appL1}\cdot\cos\varphi_1\$

\$P_{act,tot}/P_{app,tot}= \$

\$= (U\cdot I_{l1}\cdot \cos\varphi_1 + U\cdot I_{l2}\cdot \cos\varphi_2 + U\cdot I_{l3}\cdot  \cos\varphi_3)/(I_{l1}\cdot U + I_{l2}\cdot U + I_{l3}\cdot U)\$

\$= ( I_{l1}\cdot \cos\varphi_1 +  I_{l2}\cdot \cos\varphi_2 + I_{l3}\cdot  \cos\varphi_3)/(I_{l1} + I_{l2}  + I_{l3})\$

and that is not equal to \$\cos\varphi_1\$.