Legion is cool. Tali is also cool. I don't want to pick between them.
In ME2, I told the Quarians to not go to war, but they did so anyway.
It seems that it's not possible to stop the war from starting in ME3, but is there any way to end it in a way that doesn't involve total annihilation of one of the sides? I like Quarian Engineers AND Geth Infiltrators.
How do I unite the Geth and the Quarians against the Reapers?
In order to reconcile the Geth-Quarian War and gain the most out of the situation you need to score at least 5 points on the following list
Mass Effect 2 Decisions
As noted in this answer, if you did not import a game and started a new mission in ME3 you will have saved Tali, Tali will have been exiled by her people and Legion was never activated and thus the heretic mission would have have appeared. Ergo this avenue is closed if you started a new game file.
Mass Effect 3 Decisions
You must also complete Legion's Mission in ME3 or the peace options will not be available. You need a high reputation to get the Paragon/Renegade options to appear.
If you rewrote the heretics in ME2 instead of destroying them, and still manage to achieve peace, you'll get more war assets but there will be less Quarians.
The following instructions have worked well for me.
For the best result you will want to do certain things in ME2 and in ME3 when you do Rannoch. In ME2, you will need to get Tali found innocent in her trial, Destroy the Heretics in Legion's loyalty mission, and resolve Tali's and Legion's argument once their loyalty missions are done. In ME3, save Admiral Zal Koris over the crew of his ship, take out the fighters, side with Admiral Han Gerrel when he is arguing with Admiral Ran on the Normandy over the radio about their portions of the fleet, Side with Tali when Admiral Xen wants to use some of the fleet to go through a Geth debris field for tech instead of saving escape pods. With those things done, you'll want to side with Legion and then when given the blue and red options, pick one of those to persuade the Admiral Han Gerrel to stop for a minute.
I got to this point with high reputation (>400 paragon) and +7 points worth of decisions across ME2 and ME3, and the compromise conversation options still did not appear.
So I opened up my save game in this save editor and the field for Rannoch points was set to 0 in spite of all the previous decisions:
https://www.moddb.com/games/mass-effect-3/downloads/mass-effect-3-save-editor1
It seems like the game bugged out somehow. I set the value back to 7 manually and then the compromise options appeared as expected.
