Phase-field method is a verstile tool for simulation of phase-transformations in materials science. Study of the solidification in multicomponent alloys has interst in industrial applications. In the present work, MPI+OpenCL based GPU accelerated 3-D multi-component phase-field model was developed. We study isothermal solidification of ternary alloys (Ni-Cr-Nb and Ni-Fe-Si) applied to undercooling studies and constrained solidification applied to additive manufacturing in Ni-base alloys. Undercooling experiments were carried out with flux undercooling technique and in-situ solid liquid interface velocity measurements were carried out with a high-speed camera. Phase-field simulations were carried out for different undercoolings to understand the microstructure evolution. Analytical calculations based on the modified dendrite theory were used to compare with experimental results. Solidification during additive manufacturing was studied under the imposed thermal gradients and interface pulling velocity. The dendritic/cellular microstructure was simulated for various process conditions and compared with expermints. The primary dendrite arm spacing and solute segregation from experiments and simulations are in good agreement. Secondary phase evolution and cracking susceptibilty were predicted for different process conditions. Polycrystalline simulations were carried out to understand the effect of the dendrite orientations on microstructure evolution.