How does the Three Phase Transformer(Two Windings) calculate core-flux?

The help file for the three phase transformer (two windings) block states that "This block implements a three-phase transformer using three single-phase transformers". Reading this, I picture three single-phase transformers each with their own core. However, the block responds in a way such that the flux for each phase are related (i.e. total flux has to equal zero) as if there exists a shared core. During energization, the fluxes will instantneously move from the initial conditions to an arrangment where the total flux at any instant is zero. I am unable to find any documentation on how the fluxes are being calculated.

In short, how exactly is the flux in each phase being calculated?