The 'Unicab' service ran on Wednesday 26 October from outside Monument tube station, Thursday 27 October from outside Southwark tube station, and Friday 28 October outside Canary Wharf DLR station.
Created by Three's PR agency Cow to "cheer up down-in-the-dumps Londoners" as winter approaches, the service was part of the Three and ZTE's #MagicTogether campaign, which marked the launch of the ZTE Blade V7 device.
However, the activist groups deemed the campaign cruel, arguing on Facebook that "using and abusing" horses as a gimmick to sell phones was "appalling". The groups also staged a protest outside a Three mobile phone shop in London on Friday (28 October).
In response, Three apologised for any offence the activity may have caused, an added that a qualified vet had ensured the horses were "provided with the utmost safety and care" at all times.
"The horses pull carriages carrying members of the public on a daily basis as part of their regular work. The horses worked for less than four hours in total each day, were given regular breaks to rest, eat and drink, and their welfare was the main priority for all involved," a statement from Three said.
Despite the protests, however, the activity appears to have been largely successful. According to Cow PR, 97 per cent of the response on social media was positive, while coverage generated throughout news outlets was also almost entirely (95 per cent) positive.
"A no point did anyone that got a Unicab ride complain about the service," said Cow PR MD Sian Morgan.