I love TwiDash, but I also like TwiJack and sometimes AppleDash. Combining the three into one seems logical. While the story itself is not terrible, and the idea behind it is decent enough, there were some technical, story, character, and stylistic issues that really kept me from getting into the story.

Characterization:

This is focusing just on how the characters were portrayed. I'll save the romance for the story section, since that is the meat of the story.

Twilight is... Well, she's Twilight. She's acceptably so, but nothing about her portrayal really let me hear the dialogue in her voice.

Sometimes, I heard Rainbow's dialogue and thoughts in Twilight's voice, and two months is definitely too short a time for Rainbow to be picking up Twilight traits like that, except perhaps as joking or sarcastically.

Also, Rainbow Dash, as portrayed, might not be as upset, or should hold onto her anger longer than she does. A failed relationship is something that sticks with a person, and even the suggestion of getting into another one might send her into a right rage. Or, having been in one before, she might think it's not a terrible idea to try again with friends that are already friends with each other.

This kind of characterization is up to the author to decide, but it doesn't feel like a decision was made - it feels like both choices were used.

I don't really have anything to say about her character, because I was too distracted by the way that her dialogue felt like it was Ctrl+F and replaced all of the "I"s in Applejack's dialogue with "Ah". That really, really kept me from hearing anything she said in any kind of a voice other than a dentist. I know there's some that can pull this off, but unfortunately for this story, the "Ah" really kept me from hearing anything Applejack had to say.

A suggestion I would have for the author is to research Applejack's dialogue on mlp.wikia.com's transcript section and pay close attention to her word choice and phrasing. Instead of relying on phonetic spellings like "Ah" and "ya" exclusively to sell her accent, think about what word choices she uses. She does tend to use a different lexical structure than her friends.

Story

The pacing for this story was rather wonky. The part leading up to Applejack and Twilight's confrontation in the first part of the story is decently paced, with not too much happening. The inner monologue feels rather info-dumpy, however, and slows the pacing further than it maybe should have.

From there, into the conflict, the pacing shifts into over-drive, with Rainbow Dash overcoming the primary conflict promised in the description within the span of a few paragraphs and leading her to try and convince Twilight that a polygamous relationship is super-duper-a-okay where she was initially ready to do... "something" to AJ in retaliation not eight paragraphs earlier.

The conflict honestly feels a little tacked on, and barely explored, and thus it feels rushed and poorly explained.

A deeper exploration of what Rainbow Dash's past relationship meant to her, over the course of several scenes, would have made the transition much more palatable and believable.

Overall, the structure of the story was fine. The events moved forward in a clear progression from one to the next, but it felt rushed. As it is, it feels like an outline for something larger.

The plot, also, was fine. It's not necessarily a different take on a third wheel relationship, but it is decently uncommon enough that were the pacing worked on, and the details of the plot expanded, this could be a great story.

Nothing to say here, nothing lorewise was touched on.

Technicals

There are a lot of missing periods, commas, and other punctuation. There are no punctuation marks used where narrative precedes dialogue. At all. A period is required where an action is performed, and a comma where a saidism is used. A colon is also fairly common when using a saidism like:

Saidism precedes dialogue:
And then she said: "Something"

Action precedes dialogue:
And then, she did something. "You know..."

Saidism breaks up dialogue:
"You know," she said, "I wonder what it's like."

Adverbs were used quite a lot throughout this story, and places where an adjective could have livened up and added color to the story were missing them.

There is a lot of telling going on, and what showing there is is rather bland and unremarkable, and didn't capture the emotion of the scenes very well.

I would recommend looking over other romance stories and seeing how they handle adjectives, dialogue, action and describing things. Harlequin romance novels tend to be interesting fonts of ideas, but I would not recommend going full Harlequin. That's purple prose territory.

Overall

The story could have been a very interesting look into an AppleDashLight romance, but as it is, it tells the story far too quickly, and seems to want to jump right into the polygamy right after it's plot convenient for Rainbow Dash to be okay with it. The grammatical issues, and the Applejack phonetics also make this a very hard story to get into. The punctuation is a little more forgivable, but the show/tell balance is way off into Tell territory with little to no showing where emotional depth is required to make the story believable and interesting to the reader.

Rating: Needs Work