Part of ME3's problem was that it was left with too much to do, the Reaper invasion needs to be started and concluded in the same game when it should have been started in ME2.
ME1 the Reapers are unknown, they fight via subterfuge while utilizing the Geth and Indoctrination. Only one Reaper is in the Milky Way, and only for oversight. The game ends with the DLC in which the protagonist annihilates the main Relay which the Reapers wouldve used for their invasion.
ME2 starts with the overseeing Reaper killing the protagonist. Who is the nrebuild by a clandestine human supremacist organization. Its central theme is trying to rally a defense against the upcoming invasion. It ends with the "Suicide mission" against the overseeing Reapers base to forestall the creation of a Human Reaper. That was actually the first part where I got completely stumped back then: why does a human reaper matter?. It was a very good scene, but lore wise it was very confusing to me.
ME3 starts with the arrival of the Reapers and tells the story of their loosing battles, until the final attempt building on the previous cycles survivors magnus opus.
You could've split ME3 into two parts for sure, former being the loosing battle lines and the third one the final conclusion via the Ansible (i think thats what it was called?) - but I feel that last installment wouldve been too little content, if attempted. It wouldve been a good choice as a seperate DLC however, so that the main game ended with the discovery of the schematics and the DLC being about the final desperate struggle to get the resources before all is lost in the inevitable defeat.