I can't read President Trump's mind but I can take an educated guess at why he hasn't fired Rosenstein, Sessions, and Muledeer.
In business it would have been a no-brainer but in politics one has to consider all those who can and will work against you as well as the optics. Business is cleaner and more direct. Politics is messy, even bloody, plus there's the deep state swamp to consider on top of the rest. If Trump had the support of the GOPe RINO's, which he most definitely does not, it might be possible to go ahead and make these firings. Trump has a knife to his back, one to his side, and then there are his sworn enemies (politically); the radicalized dems who are standing right there with a machete in each hand. In front of Trump is a tight wire he must navigate to get to the other side. Just like he wasn't a politician until recently; he now has to develop the skill to be a tightrope walker and chew gum at the same time.
Slow moves FRiend. Slow moves until the time is right.
Yet he hasn’t been afraid of optics, the press or others. He’s largely been fearless when it comes to adversaries.
Stop! Stop! You're making too much sense for the drunken barflies to process! It's so much easier for them to blurt out between quaffs of beer how Trump "has the power to stop it.......but won’t."