It's been described as "extreme" and "crazy" on social media.
The two-year deal, which includes unlimited data and talk time, is for the largest version of the device. The same contract with an iPhone 8 costs £44 per month, plus a £99 upfront fee.
The handset is sold by Apple for £1,149 but under Three's deal customers could end up paying more than £2,100, based on its current sim-only charges.
Three's current all-inclusive sim-only deal is £29 per month.
The firm said it had various tariffs to suit different customers.
"There are a small amount of people that prefer not to pay an upfront cost yet want access to all-you-can eat data, minutes and texts and this is aimed at them," Three told the BBC.
There are cheaper alternatives, it added.
Mobile companies overcharging customers
Apple iPhone X: The internet reacts
"If that was an electrical retailer's 'interest rate' there'd be outrage," tweeted Michael Firth.
"No phone is worth over £30 [per month] in my opinion," tweeted The Cyber Heartbeat.
Smartphone price barriers
Other operators are offering the device on a monthly contract ranging between £80 and £88 but are also often charging an additional upfront payment.
O2 is charging £86 per month plus a one-off £119.99 for a similar deal with 50GB data on the larger phone.
Vodafone does not offer unlimited data either but its most expensive tariff is £80 per month plus a £100 charge for 60GB per month of data.
The phone is currently available to pre-order only.
Apple's pricing for the iPhone X had already shifted smartphone price barriers, said analyst Kester Mann from CCS Insight.
"However, Three's bold move to break through the psychological £100 per month barrier is a huge ask even for its most dedicated and loyal fans," he added.
Technology journalist Kate Bevan said it is generally cheaper to buy a handset upfront rather than as part of a contract deal.
"I'm sure the iPhone X will be a very good phone," she said.
"However, if I were going to buy one - which I'm not - I would hold off until the new year if possible, to give Apple a chance to iron out any wrinkles with it and to see if prices from mobile providers come down a bit."
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