The program is on SageMath and need Maple being coupled with SageMath.
It uses the Anders Buch's Quantum Calculator. So, we need the file qcalc in the curent directory. 
It allows to compute the divisor BPi when G/P is any classical Grassmanniann.
It also allows to compute the dimension of $H^0((G/B)^3,L)^G$.

Use:

STEP 1 -- Choose the Grassmannian to work with

> sage : load('compute_BPi.py')

Enter the Grassmannian you want to compute with. Use the quote for the name. Example:

> Enter the type of GP (Gr, OG or IG) : 'IG'
> Enter k: 2
> Enter n (it has to be even for IG):  6


Means the Grassmannian of isotropic C^2 in C^6 and hence is in type C_3.

Output : type of the group and  list of Schubert classes with their
degrees.


STEP 2 -- Compute a B_Pi

> sage : BPi(I1,I2,I3)

Entries : I1, I2, I3 are three index sets as Sage lists.
Output : 1) c(v_{I1},v_{I2},v_{I3}).
              If c(v_{I1},v_{I2},v_{I3})<>0,
	      2) the divisor B_Pi in the basis of fundamental weigths.
	      3) Check if the Schubert triple is Levi-movable
              4) the tensor multiplicities corresponding to  B_Pi and 2B_Pi
	      5) If c(v_{I1},v_{I2},v_{I3})=2, the tensor
	      multiplicities corresponding to  B_Pi/2
	      6) Check if B_Pi belongs or not to the face associated
	      to the Schubert problem

OTHER 1 -- Compute all the triples of Schubert classes with fixed value c=c(v_{I1},v_{I2},v_{I3})

> sage : SchubertCoefFixed(c)


OTHER 2 -- Compute a multiplicity for the tensor product decomposition

> sage : dimInv(theta)


-----------------------

One can also:
The output will be a list of Index Set corresponding to structure constants of H^*(G/P) equal to 2.


For each one of these Schubert triples, the program compute:
    0- Check if the Schubert triple is Levi-movable
    1- the diviseur B_Pi/2
    2- the tensor multiplicities corresponding to  B_Pi/2 and B_Pi
    3- Check if B_Pi belongs or not to the face associated to the
         Schubert problem

You can also the function indepently (once the homogeneous space
fixed).
Then, to compute BPi:



The output will be B_Pi expressed is the basis of fundamental weights.
To get the dimension of $H^0((G/B)^3,L)^G$ :

sage : dimInv(theta,3,'C')

where 3 stands for the rank, C the type of the group and theta is a (3*r)-vector or list concataning the coordinates of three dominant weights in the basis of fundamental weights.