Lots will be drawn on the night to decide which of the candidates from smaller parties, or who are running as independents, will fill his panel slot.
The same procedure will be followed for any other candidates from the five main political parties – Conservatives, Labour, Liberal Democrats, UKIP and the Green Party – who fail to show up, although all but Tory Mr Johnson are booked to attend.
Furthermore, all non-participating candidates will be allowed two minutes in which to address the audience at the end of the event.
Organiser and chair Rich Blake-Lobb, a minister in training at the church, said he wanted to ensure the debate was as “open, fair and accessible as possible”.
He added: “If there weren't so many candidates we would have loved to have had everyone, but we don't have the space.
“We're not interested so much in who people vote for as we are in making sure people get a voice.
“I want to know, as much as the next person, who I'm voting for and what the options are.
“It will be an interesting evening whether we have five main candidates or anyone else and I think it will add to the debate and discussion.”
Uxbridge & South Ruislip candidates
Those contesting the seat include three independents – Michael Doherty, Jenny Thompson and James Jackson – as well as representatives from parties ranging from the Monster Raving Loony Party and the Eccentric Party to the Communities United Party and The Realists' Party.
At the last hustings, held in Uxbridge on April 10, independent candidate Mr Doherty was not allowed on the panel.
He ended up having to leave to the back of the room, saying: “That's democracy at work here in Great Britain.”
Mr Doherty, who is running on a campaign against corruption, told getwestlondon: “It's clear that they didn't want an alternative voice or point of view that they don't know.
“It's good that there's going to be open debate at this next hustings. Independent candidates should be part of this so the public can hear all views about what's going on.”
Mr Johnson, who is tipped to win the constituency seat, also missed the first hustings, because of a mayoral engagement he was legally obligated to attend, but took part in the second.
His opponents have accused him of taking voters for granted and treating them with contempt.
The second hustings differed from the others in that all questions were pre-submitted and panelists were allowed no back-and-fourth between one another.
At the third and final hustings, all but one question will be asked by the audience on the day and candidates will be allowed to respond to one another's answers where appropriate.
Audience places will be allocated on a first come, first served basis on the day, with capacity for around 100 to watch.
Doors will open at 7pm, before proceedings get under way at 7.30pm, ending around two hours later.