The trio is knotted at 13-under-par 203 and owns a one-shot lead at The Glen Club.
Overnight leader Omar Uresti had 10 feet at the 18th hole to join the group in the lead, but missed. He shot a 1-under 71 on Saturday and is tied for fourth place with Roland Thatcher (67) and B.J. Staten (69) at minus-12.
Day had an up-and-down round going on the front nine with three birdies and a double-bogey. The horn sounded for a one-hour weather delay as Day was headed to the turn.
The break did nothing for Day, who bogeyed 11 and 13.
'After the delay, I got cold,' acknowledged Day, an Australian.
He changed golf balls after the bogey on 13 and it paid off. Day drained a 35-foot eagle putt at the 14th, sank a 15-foot birdie putt at 15 and rolled in a 25-footer for birdie at 16.
Day made a nice par save at the 17th, then hit a spectacular iron shot to 18 feet at 18 to set up an eagle try at the closing hole. He missed that putt, but tapped in for a birdie and a share of the lead.
'This week I've played well and kept my emotions pretty level,' said Day. 'I'm going to try and do that tomorrow and see how it goes. I'm going out there and enjoy it like I have the last three days.'
Riegger, a former PGA TOUR player, was flawless on Saturday. He recorded a pair of front-nine birdies at one and four, then parred every hole until another birdie at 12.
He holed a clutch 4-foot par save at the 16th, then hit a beautiful tee shot to the par-3 17th. Riegger tapped in for a birdie to get to minus-13, then elected not to go at the 18th in two, citing a back injury after the round.
'I had two yards farther than I had yesterday and yesterday I hit it six feet and made eagle.' said Riegger. 'Because my back was a little tight, I decided to lay up.'
Riegger still gave him a 7-foot chance at birdie, but missed and stayed in a tie for the lead.
Ridings, the only co-leader with a Nationwide Tour victory, tallied three birdies in his first 11 holes. He parred his next six and needed a birdie at the last to join the fray at minus-13.
He missed the putting surface with his second, but chipped to two feet. Ridings kicked in that birdie putt to reach 13 under par.
Kris Blanks (67), Jay Williamson (68), Paul Claxton (68), Dustin Bray (70), James Driscoll (71), Brad Elder (70) and Nicholas Thompson (71) share seventh place at 10-under-par 206.