Unfortunately none of the Features are rewriteable or updateable from within a program.
The best way to go about this would be to write a function to generate a plane using the 3 points you record, which should be fairly easy math.
(Planes, similar to waypoints are represented as a pose with respect to the Base frame of the robot and represent a infinite XY plane)
After that you can use the pose_trans function to move with respect to the new custom plane you have created.
Thanks, very practical information for programming "along plane" movements.
By the way, is there any lists of script functions for Polyscope?
UPD: Tried to get the robot into a caculated pose
Code
xa = ... (some calculations, based on three points' positions)
ya = ...
za = ...
rxa = ...
rya = ...
rza = ...
var_1:=p[xa,ya,za,rxa,rya,rza] // got the position in base coordinates
var_1 //move into pose
but I got the "SyntaxError: line 1 def"...