Thank you roberto . just one thing i want to ask as in "3 theta 0 90 0 (here is zero, but usually there is an offset distance, choosing alpha +90 makes J3+ to move the forearm upwards) "
i am having confusion i have compared my robot arm with many others and i found its resembles with KUKA Robot arm KR 210. It is some hoe same to it. i just want to confirm about the theta 3 as i have calculated myself by concerning many papers and videos i found it either theta 3 -90 or + 90 depending on the orientation of the arm.