West Brom were ahead twice in the game only to be pulled back on both occasions before Inamoto struck nine minutes into extra-time.
Robert Earnshaw gave the Baggies the lead after only three minutes and Luis Boa Morte equalised 18 minutes into the second half. Kanu thought he had won it for West Brom when he struck with only two minutes remaining but Helguson scored in injury time to force the extra 30 minutes.
Aston Villa avenged for their 3-1 defeat to Burnley at the same stage last season after beating the Championship side 1-0 at Villa Park.
Kevin Phillips’ goal mid-way through the first half sealed the victory in what was a disappointing spectacle which produced few chances for either side.
With no urgency and passes going astray, the game failed to produce the quality expected from the Premiership side and in the end, one goal was enough for Villa.
The goal came after 22 minutes, Kevin Phillips linking up well with James Milner to slot home past Brain Jensen.
Luke Moore had the best chance to double Villa’s lead late on when his shot was tipped round the post by Jensen, but the chances were at a premium as Villa comfortably held on.
Reading equalled a club record 17-game unbeaten run thanks to the two Dave Kitson goals that knocked Championship promotion rivals Sheffield United out of the Carling Cup.
Kitson struck twice after the break to send Neil Warnock’s understrength side back to Yorkshire to concentrate on a push for the Premiership.
The striker opened the scoring in the 53rd minute with a smart first-time finish from a free-kick move.
Stephen Hunt whipped the ball in from the left, Ivar Ingimarsson headed into the on-rushing striker’s path and he buried it past the helpless Barnes.
Kitson bagged his second and the clincher in the 76th minute, extending a leg to poke home a ball across the box from Obinna after the United back line had struggled to cope with another Hunt delivery.
Wigan needed extra-time to see off Watford, with a brace from Andreas Johansson and a penalty from Ryan Taylor nudging the Premiership side into round four.
League Two strugglers Mansfield’s gallant Carling Cup run was ended by a spectacular 90th-minute Dave Livermore winner as Millwall made heavy weather of progressing into the fourth round.
The Stags had disposed of Championship sides Stoke and Southampton in previous rounds but fell behind to goals from the Lions’ Ben May and Paul Robinson either side of half-time.
Two goals inside 60 seconds from Simon Brown and Richie Barker hauled the underdogs back on terms before Livermore’s last-gasp volley sealed the tie.
Paul Heffernan came off the bench to win the battle of the Coca-Cola League One giant-killers as Doncaster reached the fourth round for the first time in 30 years.
The Irish striker scored twice in the dying minutes as extra-time looked inevitable despite the home side’s dominance.
Heffernan, who had not been long on the pitch got Doncaster’s first after 84 minutes when he planted Guy’s cross in off the far post, and he made sure of the win two minutes from time with a neat finish into the far corner.