Gendo's own scenario for the Third Impact was "the forbidden fusion of Adam and Lilith", his goal was to be reunited with his dead wife Yui who died during the Contact experiment which lead to the Second Impact, for this he required 2 things, Adam and Lilith.
In the aftermath of the Second Impact, Adam's Soul was separated from it's body which had been reduced to an embryonic state, Adam's Soul was recovered and stored in the 17th Angel Tabris (Kaworu Nagisa) which SEELE sort to use in instigate their Third Impact by having him make contact with Lilith, if it had worked the result would be an Adam Dominated Third Impact under the control of SEELE.
One reason why SEELE couldn't send Tabris quite possible because he wasn't ready to be used yet but the other main reason was the Lance of Longinus, so long as it was in Lilith the Third Impact couldn't start (it was placed there to stop it but in a way that they could remove it one all the components for their plan were prepared), it wasn't removed until it was used as the last ditch effort to kill an Angel which no EVA could reach, it's not known how SEELE would have acted had Gendo not ordered the lance to be used.
Rei's purpose was all Gendo's Third Impact Scenario, not SEELE's. Gendo made SEELE think he was on their side however he was working on his own plans to be reunited with Yui, who Rei is a clone of. much like with SEELE's plan, Gendo couldn't act while the lnace was in Lilith however when it was removed and he could confrim that SEELE couldn't interfere with another Adam he executed his plan (which we don't see until End of Evangelion). Gendo's plan was to control Rei in a Lilith Dominated Third Impact but Rei rejected Gendo and took the embryo of Adam from him and fused with Lilith.