Flanker Seymour, 27, and centre Joseph, 20, take over from Waldrom and Matt Banahan (Bath Rugby), who have been added to the senior squad training for the opening match of the RBS 6 Nations at Murrayfield.
Benjamin, 23, replaces Delon Armitage (London Irish) following the latter's suspension from the Saxons Elite Player Squad. The Worcester wing scored five tries in two Churchill Cup appearances against the USA and Canada last summer.
Saxons coach Jon Callard said: "Dave Seymour's been playing very well for Sale and captaining the side too while Miles played a big role last summer. I'm sure he was disappointed not to be part of the Saxons EPS and this is a great opportunity for him to show what he can do.
"Everyone's delighted that JJ (Joseph) is playing again. It was always our intention to involve him at the right time, when he was fit and with his club's blessing. He's a really exciting talent."
Callard added: "The energy and attitude of the group over the last week has been outstanding and now it's important that we keep on working hard and improve our execution going into tough away game."
The Saxons are training at the National Sports Centre at Bisham Abbey and travel to Scotland on Wednesday.