Solve[ alpha c1 -beta c1^
2 -gamma c1 c2 +epsilon c3 == 0,
-gamma c1 c2 + (epsilon+theta) c3 - eta c2 == 0,
gamma c1 c2 + eta c2 - (epsilon+theta) c3 == 0 ,
c3,c2,c1 ]
{{c3 -> (c1 (-alpha + beta c1 - alpha c1 epsilon gamma epsilon eta - c1 gamma theta 2 beta c1 epsilon gamma epsilon eta - c1 gamma theta alpha c1 gamma theta epsilon eta - c1 gamma theta 2 beta c1 gamma theta epsilon eta - c1 gamma theta c2 -> (c1 (alpha epsilon - beta c1 epsilon + alpha theta - beta c1 theta)) / (-(epsilon eta) + c1 gamma theta)}}Simplify[%]
c1 (-alpha + beta c1) (eta + c1 gamma) {{c3 -> --------------------------------------, epsilon eta - c1 gamma theta c1 (alpha - beta c1) (epsilon + theta) c2 -> --------------------------------------}} -(epsilon eta) + c1 gamma theta