So, all three of those are fine, low fee index funds. But they’re all just smaller slices of the same pie. It’s kind of like saying “I want to collect all the Simpsons characters so I’m going to buy a Homer, a Marge, and a Simpson kids”. But right next to those on the shelf there’s a “Simpson family complete set”. When you buy them individually, you’re just adding complexity, and possibly missing out on part of the set (Maggie anyone???)
So back in the non-metaphor world, why buy one set of large companies, one set of medium companies and one set of small companies, when you could just buy one set of ALL companies.
In Fidelity world that would be FSKAX.
As evidence, FXAIX (large cap) contains 509 companies, FSMDX (mid cap) has 815 companies and FSSNX (small cap) has 2,012. Add those together and you get 3,336 companies. FSKAX has 3,428 companies. The slight difference doesn’t really matter (possibly due to a small gap between mid and large or who knows), but you’re basically adding complexity for no reason. And one of PFC’s Principles is that “simple is better than complex”. i.e. you’ll be more rich if you do it the simple way.
THAT said, all three of the index funds you mentioned are exclusively US companies. Some investors think it’s fine to focus 100% of their portfolio on the US, but I personally favor adding an international and bond portion. You could do that by adding FSPSX (international) and FXNAX (bonds).
And to make it EVEN SIMPLER, you can buy all three of those index funds (US, international and bonds) inside a single target date index fund that automatically rebalances and reallocates for you as you age.