ContourGF[A_[x_,y_]]:=(A^δ)[x,y]δ[x,y] + Θ[x,y](A^(g ))[x,y] + Θ[y,x] (A^l)[x,y]

ContourGF[A[t1,t2]]ContourGF[B[t1,t2]]ContourGF[C[t2,t1]]//Expand

%//.{
δ[x__]:>0, 
((Θ[x_,y_])^(n_))/;n>0->Θ[x,y],
Θ[x_,y_]Θ[y_,z_]->Θ[x,z],
Θ[x_,y_]Θ[x_,z_]->Θ[x,Max[z,y]],
Θ[x_,y_]Θ[z_,y_]->Θ[Min[x,z],y],
Θ[x_,x_]->1
}