Punters are struggling to split the three Sydney Cup contenders from the Chris Waller stable and the trainer is equally reluctant to put one above the others.

The up-and-coming Libran, a stayer from England, and Who Shot Thebarman, an Auckland Cup winner and Melbourne Cup placegetter are the $3.50 equal favourites for Saturday's 3200m test at Randwick.

The least-favoured of the Waller trio at $4.20 is the horse that won the race last year, Grand Marshal.

Who Shot Thebarman and Grand Marshal filled the minor placings in the Group One BMW (2400m) behind stablemate Preferment, the second favourite for Saturday's $4 million Queen Elizabeth Stakes.

Libran has earned his place in the Sydney Cup field with three recent wins including last Saturday's Chairman's Handicap.

"The form from the BMW stacks up time and time again," Waller said.

"Libran is the new kid on the block and he keeps stepping up.

"Who Shot Thebarman is the benchmark. He is the one whose been there for a long time.

"Grand Marshal is well weighted and ran great sectionals in the BMW.

"He's a year older and more mature but they've all drawn well and I don't think there's much between them.

"But it's still a hard race to win."

Who Shot Thebarman will carry top weight of 57kg with Grand Marshal on 55kg and Libran 53kg.

The Waller three are the only horses under double-figure odds for what is the only 3200m race other than the Melbourne Cup run at Group One level in Australia.

Libran is raced by Harry Herbert's Highclere Thoroughbreds with his owners split between the United Kingdom and Australia.

He finished fourth in the prestigious Duke of Edinburgh Handicap at Royal Ascot in June last year.

That race was won by Arab Dawn who was imported to Australia by Godolphin and won the Listed Parramatta Cup in February but after two unplaced runs, has not made it to the Sydney Cup.