Ivica Olic looks to be on his way back to Hamburg, with Andre Schurrle increasingly likely to join Wolfsburg.
Centre-back Hall, 21, and midfielder Luongo, 20, arrive from Tottenham on season-long loan deals while highly-rated Portuguese winger Reis has penned a two-year deal with a further 12-month option.
Hall joined Spurs from Brighton last summer but is yet to make his first-team debut for the Premier League side, while Luongo is no stranger to the Robins having enjoyed a brief loan spell at the County Ground towards the end of last term.
He enjoyed nine appearances for Town and scored twice, including the opener in the Wiltshire club's play-off semi-final first leg against Brentford.
Reis, meanwhile, predominantly operates down the left flank and will look to add a fresh attacking dimension to Swindon's push for promotion next term.