Cut the chicken breasts in half (to make smaller portion sizes) and season both sides with salt, pepper, and Italian seasoning.
Press SAUTE on the Instant Pot and toggle to MORE. When it reads HOT add the oil and allow the oil to get hot. Place half the chicken in the Instant Pot and cook for about 3 to 4 minutes per side, until golden brown. Transfer to a plate and set aside. Repeat with remaining chicken, adding additional oil, if needed. Transfer the 2nd batch of chicken to the plate and set it aside.
Add the butter to the Instant Pot and when melted, add the mushrooms and garlic. Cook, stirring, for 2 minutes or until the garlic is fragrant and the mushrooms have softened just slightly. Add the chicken broth and cook, stirring to scrape up any bits from the bottom of the pot. Press CANCEL.
Put the chicken back in the pot along with any accumulated juices. Place the potatoes and carrots on top of the chicken. Secure the lid and set the vent to SEAL. Select the PRESSURE COOK/HIGH setting and set the cooking time for 10 minutes. The Instant Pot will take around 8 to 10 minutes to come to pressure and start counting down.
When the cooking time completes, press the vent to do a quick release of the steam. Carefully remove the lid and use a slotted spoon to transfer the veggies and chicken to a bowl and set it aside.
In a small bowl or measuring cup, whisk the heavy cream with the Ranch seasoning and cornstarch.
Press SAUTE and whisk the cream mixture into the sauce in the Instant Pot. Cook, stirring, until thickened. Stir in 2 tablespoons parsley. Press CANCEL.
Return the chicken and veggies to the Instant Pot and stir to coat well with the gravy. Garnish with the remaining 1 tablespoon parsley and serve.