Alex Pritchard, Corry Evans and Callum Doyle have all joined the Black Cats this summer, but the club’s recruitment team continue to work on a number of new deals.
Full-backs remain a clear concern, even if Denver Hume appears to be closer to agreeing a new deal.
Head coach Lee Johnson is expected to try and sign another central defender, while Charlie Wyke is yet to be replaced.
The head coach says a number of factors have created a challenging market, but insists the club's patience will be rewarded.
"We've got a bit to do, no doubt about it," he said, speaking after the pre-season win at Hearts.
"It has been a tough market and I think we've done exceptionally well with what we've done.
"You say three signings but I 100% see it as five with McGeady and O'Nien, because they were in the open market.
"But we still have a lot of work to do, we know that.
"We clearly need a left back, a right back. We have got players who can play there, but we don't want to take players away from their natural position. We know that.
It has been a tough market because you've got GBE to account for, which means foreign players that used to be available are not.
"Then you've got the Premier League purchases, if you like, from the U23s and the lads who've been out on loan. We're willing to pay fees for these players but they often come with extremely high wages and often aren't willing to take a drop.
"Then you've got the talented players from lower levels who now command higher fees because they're English, and we do the same with any of our young English players.
"The pool of players is then quite small. You've got to get the right ones and we will get them, no doubt about it."
So who could be the next new face through the door on Wearside? We take a look at the latest speculation.
. SUNDERLAND, ENGLAND - MAY 09: A General view of the Stadium of Light prior to the Sky Bet League One match between Sunderland and Northampton Town at Stadium of Light on May 09, 2021 in Sunderland, England. (Photo by Pete Norton/Getty Images)
Sunderland eyeing audacious striker swoop as Cats sent clear message over defender target - plus Sheffield Wednesday, Portsmouth updates Photo: Pete Norton
. Blackpool remain keen on Elliot Embleton
The Championship side remain keen on striking a permanent deal for the Sunderland academy graduate after his successful loan spell last season - but are yet to meet Sunderland's valuation (The Sun) Photo: Frank Reid
. Portsmouth winger puts Championship clubs on red alert
Ronan Curtis has admitted that he wants to test himself in the Championship - amid rumoured interest from Blackburn Rovers and Cardiff City (Portsmouth News) Photo: Naomi Baker
1. Blackpool remain keen on Elliot Embleton
The Championship side remain keen on striking a permanent deal for the Sunderland academy graduate after his successful loan spell last season - but are yet to meet Sunderland's valuation (The Sun) Photo: Frank Reid
2. Portsmouth winger puts Championship clubs on red alert
Ronan Curtis has admitted that he wants to test himself in the Championship - amid rumoured interest from Blackburn Rovers and Cardiff City (Portsmouth News) Photo: Naomi Baker
3. Sunderland sent clear message over defensive 'target'
Preston North End are reluctant to lose full-back Joe Rafferty this summer - and would demand a fee if they were to sanction an exit. He has been linked with a move to Sunderland (Lancashire Evening Post) Photo: Nathan Stirk
4. Cheltenham Town monitoring ex-Bristol City defender
Ex-Bristol City full-back Vince Harper is on trial at newly-promoted Cheltenham, and featured in their pre-season defeat to Birmingham City (Gloucestershire Live) Photo: Alex Burstow