Rob Holding, Nuno Tavares and Albert Sambi Lokonga all departed Arsenal on a busy day of outgoings on Deadline Day.
Crystal Palace got the deal done to take defender Holding for £4m.
Holding was made available for transfer by Mikel Arteta having not featured this season and had plenty of suitors with Luton and Sevilla among the clubs interested.
The 27-year-old defender made 162 appearances during his seven full seasons with the Gunners.
“I’m excited to get started and get involved,” said Holding.
“Playing against Crystal Palace the number of years I have, seeing the fans and how intimidating Selhurst Park can be, and how loud it can get, I’m looking forward to being on the right side of that this time!”
Tavares joins Forest on loan
Nottingham Forest reached a deal with Arsenal to sign left-back Tavares on an initial loan move.
The loan fee is £1m, with Forest holding a £12m option to make the deal permanent for a player who has two years left on his current Arsenal contract.
“Forest is a historic club which won two European Cups and to be part of it makes me really happy,” Tavares said.
“I’m really happy to stay in the Premier League as it’s the best league in the world and I’m thankful to Forest for giving me that opportunity.”
Lokonga snapped up by Luton
Luton completed a Deadline Day loan move for Gunners midfielder Lokonga, who has been linked with Brighton and Burnley this transfer window.
He was in the stands to watch his new side in action as they lost 2-1 at Kenilworth Road against West Ham.
Lokonga was not part of the Arsenal squad for the pre-season tour of the US and spent the second half of last season on loan at Crystal Palace. He made nine appearances for the Eagles, helping steer them to Premier League safety.