For comparison with other CAS choose from: Axiom Derive Macsyma Maple Reduce
    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,
            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