Coding a triple absorption model

> Dear NMusers, I am using a dual absorption model as follows (The dose is split into 2 fractions and the splitting fraction, THETA(.), is an estimated parameter in the model): TPH = LOG(THETA(.)/(1-THETA(.))) TH = EXP(TPH+ETA(.))/(1+EXP(TPH+ETA(.))) F2 = TH F1 = 1-TH However, upon prolonged sampling we can observe a 3rd shallow phase. I would like to use a 3-depot absorption process with F1, F2 and F3=1-F1-F2. Both F1 and F2 need etas using some type of transformation such that neither one of them exceed 1 nor does their sum exceed 1. Can some one kindly provide the NONMEM code. Thanking you in advance, Mahesh