ME3 does not playout differently in any meaningful way based on any choices made in ME1 or ME2. Yes you can make choices in ME3 that change how it plays out but there are few changes to ME3 based on anything the player does. And of these "changes" almost all are cosmetic and of zero importance to the ending. It doesn't matter if you cure or don't cure the genophage, it has ZERO direct effect on the ending. Doesn't matter who wins the Quari-Geth war, if the geth win or Quari or they make peace there is absolutely zero direct impact on the endings. Even saving or killing the Rachni doesn't change anything, if you save them the reapers indoctrinate them and if you don't the Reapers artificially construct rachni so you can run into these same enemies you would have otherwise. Grunt Dies in ME2 no worries the mission stays the same just with a new guy, Kill Wrex in ME1 no difference in the game except there is a different leader of the Krogen, mission plays the same. There is no major content change in Me3 from anything in ME1 or ME2, besides dialogue options. No content is gated based on the past games, you will do all the same mission in all the same ways. No mission is unlocked because you did or didn't do X and no mission is Locked because you failed to do y. At least nothing major, some minor character interactions can change but nothing of substance.
Yes the story changes based on what choices you make in ME3 but by and large almost all choices remain the same regardless of ME1 or Me2 and even when you do get a "special" option that doesn't change the endings. We were specifically told that our choices in ME1 & ME2 would change the outcome of the ending of the series. That was a major selling point of the series. It turned out to be impossible but apologists for Bioware can't even admit this glaring fact. Instead they go on and on about how there is still choice in the game and the game plays out different based on said choices. Which was never the criticism, no one is claiming there isn't choice in ME3 they are claiming none of the choices you make matter, which is a direct contradiction to what we were told.