Paceman Milnes, 22, was part of the team that clinched the 2012 County Championship after signing in 2011.
Lewis, 24, featured eight times in the T20 Blast this summer and McKay, 20, deputised for wicketkeeper Tim Ambrose.
"All three have given everything and it will be sad to see them leave," said director of cricket Dougie Brown.
"Sadly opportunities at senior level have been limited, since signing their first professional contracts, so we believe that it's in their best interests to look at new opportunities."

The union Unite organised the rally at the firm's headquarters in Derbyshire.
The union has called for an end to "Victorian working practices" and demanded staff get a living wage of Â£7.85 an hour.
Sports Direct said casual workers receive holiday and sick pay, and are included in incentive schemes.
The protest at the company headquarters was one of about 40 being held on Wednesday, with Sports Direct shops across the country also being targeted.
A Sports Direct spokeswoman said: "Much of the comment around the group's use of zero hours has been unfounded and inaccurate.
"The group complies fully with all legal requirements which relate to casual workers, including holiday and sick pay and freedom to gain other employment.
"Casual workers also benefit from general incentive schemes."
Unite said an estimated 3,000 workers are on zero-hour contracts at Sports Direct's Shirebrook headquarters.
A further 75% of staff across its UK stores are also on zero-hour contracts, with Sports Direct accounting for a fifth of all such contracts in the retail sector, according to Unite.
One institutional investor, Royal London Asset Management, earlier called for the resignation of Sports Direct founder Mike Ashley citing anger on his failure to attend four board meetings.
Ashley Hamilton Claxton from Royal London Asset Management, said: "We have lost confidence in the board and are very concerned about the long list of corporate governance failings that have not been addressed."
But both Mr Ashley and Sports Direct chairman Keith Hellawell were re-elected to the board at the meeting.
Unite representative Cheryl Pidgeon said: "We want an end to zero-hour contracts - because young people cannot stand 'forever and a day' to be on zero-hour contracts.
"We think Sports Direct makes enough money to ensure that their workers have to dignity, respect and fair wages."

The 29-year-old, from Cornwall, started his career at Penryn and has played for Exeter Chiefs and Rotherham, as well as spending time playing in Spain.
"He's impressed us - he is renowned for his work-rate, physicality and passion," said coach Gavin Cattle.
Alex O'Meara and Alex Day have agreed new one-year contracts, with hooker Tom Channon signing for two more seasons.
Wing O'Meara, 24, has spent two years at the Mennaye, while scrum-half Day has played 52 games in his time at the club.
Channon joined from Bristol in 2013 and is three games short of making 100 appearances.
Pirates are sixth in the Championship with two games to go, and now have 19 players confirmed for next term.

The 36-year-old former Watford and West Bromwich Albion defender was without a club when he joined Blues on a one-month trial deal in September 2012.
But this new contract, in a player-coach role, is his fourth for Blues as he continues to extend a career that now totals more than 700 appearances.
Robinson has not figured since being sent off at Bournemouth on 6 April.
But he has still clocked up 36 appearances this season, taking his total number of games in a royal blue shirt to 119.
"Robbo is an influential person around the dressing room and been a good help to me since I've come in," Rowett told BBC WM. "And I still think he can offer something on the pitch.
"There is an opportunity to become a squad defender and still be influential around the club and help some of the youngsters in a coaching capacity, maybe one or two days a week.
"Sometimes it's hard to find someone that can offer a multitude of things. They can help the young players with that game understanding and that knowledge of winning things and doing things properly in football, because that's something you don't often get in academies.
"Every time he pulled on the captain's armband he gave it everything he's got. He's a real winner and a real leader and sometimes I don't think there are enough of those around clubs in the game."
Warwickshire trio Tom Milnes, Tom Lewis and Pete McKay are to leave the club at the end of the season when their contracts expire.
[NEXT_CONCEPT]
A Dickensian-style protest has been held to oppose Sports Direct's use of zero-hour contracts at the company's annual general meeting.
[NEXT_CONCEPT]
Cornish Pirates have agreed to sign lock Toby Freeman from Nottingham on a two-year deal, starting next season.
[NEXT_CONCEPT]
Birmingham City club captain Paul Robinson has signed a new one-year contract with the Championship club.