I'm not realy a friend of the surface object. It creates good approximations for the input data but the outcome is not stylish:

the poles grid in this case does not have a relation to the border faces and the face is a patch of the bspline surface.
the waviness of the surface can be smoothed in a second step but it is the result of a brutal interpolation.

I think the task is to find a method which creates for a triangle gab definde by 3 nurbs surfaces a filler consisting of 3 unpatched nurbs.

in my example I used 3 ruled surfaces (along bezier curves) as borders (brown/pink ) and created inside the hole 3 nurbs red, blue green
because there is only a small number of poles and because of the simple bezier curves the continuity is not anywhere 2 but the isocurve structure is naturally. The grid of the inner faces is scaled by factor 2 to show the balanced correlation between the knots of all faces.

for statistics:

the border curves have each 7 poles (two connected bezier curves)

the inner area has 36 poles - each segment is a 4 x 4 bspline surface of degree 3, so all edges are simple bezier curves.

here is my example file

https://www.dropbox.com/s/i6n5sas1ky5c6 ... fcstd?dl=0