The star trio were notable absentees at Fremantle's open training session on Tuesday.
Mayne is carrying a sore back, while McPharlin and Johnson only recently returned from calf injuries and were somewhat hampered during last week's 25-point win over Sydney.
Fremantle football operations manager Chris Bond denied there were any major injury concerns leading into Saturday's game, and said Mayne, Johnson and McPharlin were certain starters.
"A couple of them pulled up a little bit sore in the game," Bond said on Wednesday.
"But they got through the week really well (and) doctors are under no doubt that they'll play.
"From our point of view, it's not a real threat if they just jog around tomorrow as well.
"All will be available and all will play."
Bond said Mayne was able to carry his back complaint during the game against Sydney.
"It wasn't anything major. He played really well on the weekend, so it didn't restrict him in any way," Bond said.
"It was just being cautious with him yesterday. We're expecting him to train tomorrow."
Fremantle will hold their main training session behind closed doors on Thursday morning before making the trip to Melbourne in the afternoon.
The Dockers are expected to take an unchanged line-up into the grand final.