Daizen Maeda, Yosuke Ideguchi and Reo Hatate were all signed by Celtic.
Maeda played for Celtic under the management of Ange Postecoglou.
Ideguchi, a 25-year-old, is leaving Gamba Osaka to move to Celtic, while Hatate, a 24-year-old, joins from Kawasaki Frontale.
The players that we have brought in to Celtic are players that will be exciting for our fans.
I think bringing all three to the club will bring even more quality, personality and energy to the squad and I think that will help the players who have done so well for us already this season.
"They are all at the peak of their careers,ambitious to achieve more success and I am sure at this particular time we will be getting their best years."
Ideguchi was a player for 18 months but failed to make an appearance.
I am happy to have secured these players early. I look forward to working with them, and my backroom team and the rest of the squad will be making sure we help them settle into Celtic as quickly as possible.
Maeda joined Celtic after a season in which he was the J League's top scorer with 23 goals.
The forward, who has two caps for Japan, will join Celtic on a loan and sign a long-term deal in the summer.
Reo Hatate is a two-time Japanese champion.
Ideguchi spent 18 months as a player for the club but never made an appearance, and returned to Gamba Osaka in August 2019.
The Japanese player has 15 caps and two goals for his country, and has signed a four-and-a-half-year deal at Parkhead.
Hatate joined Celtic after helping Kawasaki Frontale to their second successive Japanese title.
He was part of the U23 side that reached the semi-finals of the Olympics in Japan this summer.
The trio will be registered as Celtic players in early January, with the club hoping they can join the squad early in the New Year.
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Celtic were the opposite of Rangers. Celtic had a lot of work to do before the season started, and Rangers were settled and had the confidence of winning.
The managerial matters dragging out didn't help, but since Ange Postecoglou has come in you have to say he has been very assured with his work. He's very sure of what he wants from the team.
The signings have been good and I think if he gets the chance to add more in January, the Celtic support will take confidence from what he's already achieved.
The league is where it is decided if the campaign has been a success or not, and getting the first trophy helps.
I think Postecoglou has the ability to have a really strong Celtic XI if given the funds to strengthen. The depth of squad is what they fall short of.
Having a strong squad is important to Celtic because of the difficulties with Covid, the Africa Cup of Nations coming up and normal injuries and suspensions.