That trio is one stroke clear of first round co-leader Raymond Floyd (73), Morris Hatalsky (69) and Isao Aoki (71). Bruce Lietzke, Craig Stadler, Tom Watson and Jose Maria Canizares share seventh place at 3-under-par 141.
Charles began his day two strokes behind Floyd and co-leader Sam Torrance. The New Zealand native quickly climbed to 5 under as he birdied each of the first two holes at The Hills Country Club.
The 23-time winner on the Champions Tour ran off four straight pars before stumbling to a bogey at the seventh. Charles tripped to another bogey at the ninth to slide back to 3 under.
After a pair of pars, Charles dropped in his third birdie at the par-4 12th. He posted four straight pars before grabbing a share of lead with a birdie at the par-4 17th.
Charles is trying to become oldest winner on the Champions Tour. Mike Fetchick currently holds that mark having won the 1985 Hilton Head Seniors Invitational at the age of 63. Charles is also trying to win a professional event for the sixth straight decade.
'This is the type of course I feel comfortable on, but I'm trying not to think about winning,' said Charles. 'That will just put pressure on myself. I can't believe no one got the better of the golf course today.'
Levi picked up his first birdie at the par-4 fourth. He later eagled the par-5 eighth to move to 5 under. He stumbled to bogeys at the ninth and 12th. The 2003 3M Championship winner bounced back with birdies at the 14th and 18th to share first place.
'You have to commit to your shot in winds like this, but that's been the strength of my game,' Levi said. 'I hit the ball really solid today. I haven't had as many high finishes because I haven't made many putts.'
McNulty, who won the Outback Steakhouse Pro-Am earlier this year, began the day at minus-4. He climbed out in front with birdies at the second and fourth.
He moved to minus-8 with an eagle at the eighth. However, things went down hill from there. The Zimbabwe native faltered to a double bogey at the very next hole. He fell back into a share of the lead with a bogey at the par-3 16th.
'I was just late on my swing and that's what happens,' said McNulty of his double bogey. 'I came back nicely, but it was a disappointment. I think any score under par today guys have to be pleased with.'
Larry Nelson carded a 3-under 69 to move to 2-under-par 142. He is joined there by D.A. Weibring and Tom Kite, who both shot 73s Saturday.
Torrance stumbled to a 4-over 76. He stands at 1-under-par 143 where he is joined by Ed Fiori, John Harris, Joe Inman, Graham Marsh, Dana Quigley and Bobby Wadkins.