Mass-effect – If you increase galactic readiness using multiplayer, can you still get the “worst” ending to Mass Effect 3

In Mass Effect 3;

• Single Player: Has no affect on your Multiplayer besides you gaining experience with different weapons that would ultimately be harder to get in Multiplayer (i.e. Black Widow, and other weapons of N7 Quality.) Allowing you to know what weapons you want with what mods and classes.

• Multiplayer: - Has a small but noticeable Affect on your Single Player Campaign.

  Whenever you have a successful mission in Multiplayer, you will gain a Galactic Readiness Percentage increase in your Singleplayer (i.e. 5% increase to Galatic Readiness). Every player starts with a minimum Galactic Readiness of 50% (your Galactic Readiness will decay over time if you do not continue to increase it) although there are 6 sectors that make up the "Galactic" Readiness.

  Each sector has a map associated with it in Multiplayer which allows you to increase the Sectors Galactic Readiness by selecting and completing a multiplayer mission on that map. Although if you select random map, in multiplayer, and win you will achieve an increase for all the sectors Galactic Readiness. Another thing to keep in mind is the harder difficulty of Multiplayer mission that you successfully complete (i.e. Bronze, Silver, and Gold) the higher percent of increase to your Galactic Readiness you will receive.

  Your Galactic Readiness affects your war assets in single player completely. Say for example you have 1000 War Assets and only 50% Galactic Readiness, then only 500 of your War Assets will go towards your goal of achieving a victorious ending in single player. Completing a multiplayer mission while having a readiness rating of 100% grants a 5% boost in experience earned.

  Another benefit to single player from multiplayer is the option of adding War Assets to your Single Player Game by Promoting a level 20 character that you have created. This will give you 75 War Assets and this can be done as many times as you wish. This promotion also adds 10 points to your N7 Rating and allows you to start over in Multiplayer on that character.

  Doing harder difficulties in Multiplayer doesn't affect your war assets when you promote your characters. so doing gold in hopes of getting more war assets then doing bronze will only disappoint you.

Some proof for you. Hope this is what your looking for, couldn't find anything else.

BioWare has stated previously that you can gain "effective military strength" through any combination of single player or multiplayer. Single player play contributes "War Assets" while multiplayer contributes "Readiness Rating." These two things multiplied together are a big factor in your ending.

[...] SP can still be played all by itself without any kind of multiplayer or third party and you can still get the absolute best ending. [Multiplayer] just gives you new options to be able to get that ending.

Also in a different interview with Penny Arcade:

You can get best possible outcome in the game by just playing single player if you want, as long as you are willing to do a lot of the side missions and content. However, maybe you only want to do a critical path run of the single player. Well, by playing multiplayer you also increase your galactic readiness rating can affect the final outcome. Using the iPhone Datapad app and the iPad Infiltrator game will also tie into Galaxy at War and update this final rating.

However, it might be challenging to just play the single player. From this PC gamer review:

The game’s still story-driven, and it doesn’t end until you’ve completed the main series of missions. But when you do, what happens in the final cut-scene depends on how many War Assets you have accumulated.

It is possible to get the best ending in single player without playing multiplayer, but it’s twice as hard. All your War Assets only count for 50% of their potential value.

Their intent is to allow people to rush through the single player campaign (if they don't care for the story or side missions) and play a bunch of the multiplayer, and still get the "best ending."

Now that I've finished the game, there's a couple of things I'm going to add to what BioWare has said. While BioWare claims that you can get the "best ending" without playing multiplayer (or the tie-in titles for other platforms), I find this statement to be misleading at best.

It's incredibly unlikely that you will be able to get enough war assets for the "highest tier" endings to Mass Effect 3 without anything increasing your Readiness Rating. You'd have to have played ME1 and ME2 pretty thoroughly, making all the "correct" decisions in your save to import into ME3. Alternatively, you'd have to start from a "perfect" ME2 save import in order to get anywhere close to the number of War Assets you'd need. If you're not importing a previous save, your chances at playing only the single player and seeing the "best" ending are abysmal.

Additionally, you'd have to play this game through very carefully, making sure to maximize your War Assets. Most decisions in the game effect your war assets, and if you don't know ahead of time what the "best" decisions are, you're likely to end the game with significantly fewer assets than you need for some of the "best" endings.

I played fairly well through both previous games, skipped the DLC, and got nearly every war asset possible in ME3. I still fell a good 20-25% short on War Assets required for the "highest tier" ending.

Having said this, the endings are extremely similar. Regardless of how you play, you're highly likely to have very similar options in your ending choices, and each of the endings plays out extraordinarily similarly.