I want to solve these differential equations but could not get the simplified answer. Can anyone help me? it will be very appreciated.
See the attached file for your reference.
What is the purpose of this question? Mathematica finds the solution with all seven parameters C2, Bi, k, A1, S, A2, A3 free: assumption free and without values:
as usual
the simplified answer
appears with the help of specialisation. If all parameters must remain formal, then you could try to find subexpressions in the solution.