All would have been out of contract next summer but have struck deals to extend their stays at Deepdale.
Defender Hughes has signed a deal until the summer of 2022, with left-back Earl and midfielder O'Reilly signing-up until 2023.
Hughes has been a regular in North End's defence, primarily at left-back, for the last two seasons since signing from Peterborough.
Southport-born Earl came through PNE's academy and made his first-team debut in September 2017.
He had loan spells at Bolton and Ipswich last season but is now back at North End and travelled with the squad to Huddersfield last Saturday.
O'Reilly, who is currently on loan at Bamber Bridge for first-team experience, joined the academy from Irish football and turned professional a couple of years ago.
His one appearance for PNE came as a substitute against Aston Villa in December 2018.
North End manager Alex Neil said: "They are all at different stages of their careers. We have had a lot players running into their last year and it was important we dealt with that.
“Adam has gone out on loan and Josh spent last year out on loan, and these will be good experiences for the both of them and Andrew, when he has been fit, he has been a virtual ever present for us.
“For different reasons, it is good business for us to get them tied down and let’s hope the younger lads can flourish – Adam can get experience; Josh can force his way into the squad and the team and Hughesy can try and continue the path that he is on."
The trio join Ryan Ledson, Sean Maguire and Josh Harrop who signed new contracts in the summer.
Goalkeepers Declan Rudd and Mathew Hudson agreed fresh deals during lockdown earlier in the year.
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