In February, the company said it wanted to give its customers "control, choice and greater transparency" over the adverts they received.
But the Internet Advertising Bureau (IAB) said Three's approach was too "broad" and would harm businesses.
The opt-in trial will take place on a day between 13 and 20 June.
Three said it was exploring ad-blocking on its network because:
In February, it announced a partnership with ad-blocking start-up Shine, in which Li Ka-shing, the owner of Three's parent company CK Hutchison, is an investor.
The mobile network said the technology would block 95% of pop-ups and adverts on websites, but the pre-roll video adverts, sponsored articles and in-feed promotions on social networks such as Twitter and Facebook would not be blocked.
Steve Chester from the IAB said encouraging advertisers to produce a "lighter, less invasive ad experience" was preferable to ad-blocking.
"We're all committed to solving the ad-blocking issue but disagree with Three's approach that network-level ad-blocking is the way to go," he said.
"It's a broad-brush approach that the largest media owners can probably survive but not the smaller ones.
"In the long-term consumers will also lose out, as they'll most likely have to pay for services that are currently free because they're supported by advertising."
Many people already choose to block adverts online by downloading browser plug-ins or mobile apps that can strip promotional content off websites.
Ad-blocking at a network level could reduce mobile data use by customers, but could also let a mobile service provider decide who can advertise to its customers, or charge fees to "white list" advertisers.
Three told the BBC it was in the early stages of testing the technology and did not know whether it would charge customers for an ad-blocking service, but said it would not "white list" advertisers, were it to launch one.
The company said it would contact 500,000 of its customers to invite them to take part in the trial.
Mobile service provider Three has confirmed it will block advertising on its network for a day-long trial in June.