Mixed u-p formulation - ANSYS

[eLinLiN]

I would like to understand how the mixed u-p formulation in the ANSYS works.
For PLANE183 element in the ANSYS's manual we can find that:

There is a linear function for hydrostatic pressure interpolation:

P=P1+sP2+tP3

but in the table we can read that there are just two internat nodes.
I do not understand this. I would be grateful if someone explained it to me.

Thanks in advance.