The trio suffered serious side effects after taking blue tablets, known as Blue Ghost, at the music festival on the Shropshire-Staffordshire border.
Staffordshire Police said officers have seized several of the tablets.
Government scientists are at the same event in Essex analysing samples of so-called legal highs after campaigns warning people not to take them.
A ban on legal highs at the twin festival sites was introduced seven years ago.
So far, more than 120 people have received medical treatment at Weston Park in Staffordshire, with two people needing to go to hospital, ambulance crews said.
Paramedics said it had been a "steady start" for staff and volunteers since the gates opened to campers on Friday.
Staffordshire Police said the three people taken ill from the tablets did not need to go to hospital and officers were investigating the incidents.
"Emergency teams have treated three people who took a blue tablet they believed to be ecstasy at V Festival in Weston Park," a spokesman said.
"All had serious side effects after taking the tablets and needed urgent medical treatment."
Three people have been given urgent medical treatment after taking a drug they thought was ecstasy at V Festival.