Beware the three-headed monster.

That is the message from Shaun McManus to West Coast ahead of Friday night, with the Fremantle legend believing the Dockers’ trio of key forwards are primed to relish the western derby stage and continue their white-hot form against their cross-town rivals.

The improvement of Jye Amiss as a contested-marking beast was on full display as he slotted four goals in his side’s comeback win over Port Adelaide on Saturday, which boosted his tally to a career-best 43 for the season.

Josh Treacy is often deployed further from goal than his fellow key forwards but has still managed 44 majors for the year to sit third in the race for the Coleman Medal, behind only a pair of more traditional full-forwards in Sydney’s Charlie Curnow and Gold Coast’s Ben King.

Meanwhile, Pat Voss becoming Fremantle’s No.1 option to be deployed in a defensive role against opposition intercept marking threats has not quelled his offensive influence, with a one-goal effort against the Power snapping a run of at least two in each of his last eight appearances.

McManus — who will be part of Channel Seven’s commentary team for the derby — said the fact Optus Stadium would be draped mostly in purple for what is Fremantle’s home derby was not a factor to be underestimated.

“Any game they’ve been able to play at home, they’ve been able to get up and about,” McManus said.

“Patrick Voss, last week, he was pretty quiet, so I suspect he’d be looking to bounce back from a lower game. Jye has been great and so has Josh Treacy.

“They’re going to have 50,000 people getting behind them on the weekend . . . I think they’ll be able to get up for that, for sure, and I think they’ll be dangerous.

“The West Coast defenders are going to have to be on the ball . . . both those players (Tylar Young and Rhett Bazzo) have been in really good form, but they’re coming up against the best. It’ll certainly be an audit on how they’re going, that’s for sure.”

Fremantle’s stunning campaign has all but assured them of a coveted top-two finish on the ladder. They sit three victories clear of second-placed Sydney and would need to drop at least three of their final five games to end up below third-placed Hawthorn by the time September rolls around.

But by comparison to their blistering start, their form in recent weeks has only been patchy. The Dockers’ last month has included what was just their second loss to the season against Greater Western Sydney and only toppling a 14th-placed Port Adelaide side without several of their best players on the back of a second-half comeback.

McManus said Saturday’s contest against the Power would have given coach Justin Longmuir the chance to reinforce some important messages.

“They weren’t at 100 per cent from the get-go, and that’s what happens — you take on a team in an AFL game and you think it’s just going to happen and they’ll roll over, that’s not how it works,” McManus said.

“From Justin’s point of view, he’s got to look at it as he has done all season — one week at a time.

“They’re going to build it up, because it’s a game they need to win. He’ll say, regardless of whether it’s a derby or not, we need to win the game, to make sure they keep elevating themselves ahead of the pack.

“Having that three-game buffer ahead of Sydney, it is a must-win, in that way.”

Meanwhile, West Coast coach Andrew McQualter set the early tone for the derby when he declared his side would be out for a “scalp” against the Dockers immediately after their credible defeat at the hands of back-to-back reigning premiers Brisbane on Saturday.

Slow starts have been an issue which have dogged the Eagles for much of the year but they burst out of the blocks against the Lions, which McManus said would be crucial for their hopes of pulling off a shock upset of their cross-town rivals.

“I think they’re capable, with what we’ve seen over the last couple of weeks, against Brisbane and the Bulldogs in particular. They’ve shown that they can compete, it’s just how long they can compete for,” he said.

“He would certainly be looking at it in the way that if you challenge them early, you can certainly get yourselves in the game, and then, who knows.

“The pressure then goes on the team who are expected to win, against the team who are the underdogs. Momentum’s a funny thing — once they start believing in themselves, the result could really start to go their way.”