Both Roby, who won the award in 2007, and Tomkins were short-listed last season but lost out to Rangi Chase, the Castleford and England half-back.
This time they are facing competition from Westwood, who has earned a growing reputation as one of the hardest-working forwards in the English game.
The winner, chosen by a secret ballot of Super League players to reward the man deemed to have made the biggest impact on the season, will be unveiled at a gala dinner in Manchester on Monday, October 1, when the coach of the year and young player of the year will also be announced.
The nominees for young player are Bradford second rower John Bateman, who turns 19 today, St Helens centre Josh Jones, also 19, and 20-year-old Leeds full-back Zak Hardaker.
The young player is chosen by a specially-convened panel while the coach is selected by a secret ballot of all 14 Super League coaches.