Alright, on the Mass Effect 3 "do you have to play multiplayer to get the 'best' ending" issue: no, you don't. If you have imported a Shepard from the last two Mass Effects and you have made the right decisions, it is perfectly possible to unlock the "Green" ending without ever touching multiplayer. When I played the game on my 360, that was certainly the case. In fact, this is even easier to do if you download the Director's Cut Extended Ending DLC, as along with the ending changes it
also lowers the War Assets threshold you have to cross to get the "Green" ending to a much more manageable number. You certainly don't have to play the multiplayer if you don't want to.
And, incidentally, you probably don't want the "Green" ending anyway, since despite it feeling like the only ending where you aren't massively betraying everyone, nothing about it felt right to me.
As for Mass Effect 3 in general, this was the game that really just poisoned the franchise well for me, and for reasons beyond "the ending". I just found it a very cold and calculating game where choices made very little difference and almost
none of your rewards are reflected in the big battle at the end. For instance,
where are my Krogans riding Dinosaurs and Elcor marching into battle with canons strapped to their backs? 
: : : : I know, that sounds like a totally nitpicky complaint, but those are war assets I won by completing side quests. I want to
see them in the final battle,
especially ones like that that sound so ****ing
awesome. But that's Mass Effect 3 in general: promises never really fulfilled and story threads that end in a less-than-satisfactory way, culminating in a final sequence that makes
absolutely no sense (
especially if you managed to bring about peace between the Quarians & Geth not three
hours prior!). It's also worth noting that the entire Mass Effect series is about how sentient beings aren't mere numbers, & that when they stand together they can make a difference. So what's your reward for doing
anything in Mass Effect 3? People are turned into mere numbers thrown in a very visible pile. That
this game beat Xenoblade for general Best RPG of 2012 is
absurd.
Yeah, I mean to replay the trilogy someday and experience the Leviathan and Citadel DLC packs they put out to once
again try to perform damage control on Mass Effect 3. However, as of this writing I feel very unlikely to want to jump into a future Mass Effect game again. That series just feels irreparably damaged.