I have obtained some pages from the Canterbury annual report from 1976 which details the amount of games that each player played during the competition.
Anderson 3
Charlton 2
Cook 1
Dowling 3
Hage 1
Harris 3
Howard 1
Garry Hughes 3
Graeme Hughes 2
M.Hughes 3
Mortimer 3
Moseley 3
McCarthy 3
Noonan 2
O'Neill 3
Peek 2
Peponis 3
Phillis 1
Ryan 2
White 1
Most of the information can be verified but some assumptions have been used. Canterbury used 20 players in the competition who played 45 games in total. I am assuming 2 replacements in each game.
Round 1 V St.George
https://www.rugbyleagueproject.org/competitions/wills-cup-1976/round-1/canterbury-bankstown-vs-st-george-dragons/summary.html
Charlton did not play this game refer to the Sun dd 23/2 which says that he will be returning for round 2. The SMH & Sun dd 20/2 says that Mick Basham will replace Charlton, but he was not listed as having played in this competition in the annual report, but Greg Cook does. I am assuming that Cook played in this game. As Graeme Hughes played 2 games & was used in the round 2 & 3 games, Cook seems likely in this game.
Round 2 V Norths
https://www.rugbyleagueproject.org/competitions/wills-cup-1976/round-2/canterbury-bankstown-vs-north-sydney-bears/summary.html
The players you have listed in this game for Canterbury are the same that I had in my research.
Round 3 V Manly
https://www.rugbyleagueproject.org/competitions/wills-cup-1976/round-3/manly-warringah-sea-eagles-vs-canterbury-bankstown/summary.html
The only amendment I would make in this game is the addition of Steve White on the bench as the previous 2 games already had 2 replacements in each.
Thanks Ted I have updated.
Regarding round 3 Canterbury V Manly, Manly's Lowe had number 9 in big league & in the big league's review of the game, they said that number 16. Wilson came onto the field & Canterbury's White was listed as number 16.
Thanks I see that now all fixed now.