It is understood that one of the pupils has now left the 260 acre school.
A source told The Sun: "The boys went into Harrow town centre last Wednesday night and bought 20 pills from a dealer.
"They took some but they are thought to have been a bad batch of Ecstasy and they all became ill.
"There was a major panic and staff and the matron were called.
"They called an ambulance as there were serious concerns about one of the boys. He was taken to hospital where he was treated and initially detained for observation."
The school, where students are required to wear straw boater hats, has previously faced criticism for a drug culture at the school.
"The boys were all suspended by the head Jim Hawkins and the board of governors were informed," the source added.
"One of the boys has now left the school and a decision will be taken at some point on what to do with the other two boys.
"The school takes a firm stance on drugs these days but will be loath to wreck a young boy's education because of one mistake."
A London Ambulance Service spokesperson said: "We were called to reports of an incident at an address on High Street, Harrow.
"We sent an ambulance crew to the scene. We treated a patient and took them to hospital."
A spokeswoman for the school said: "Our policy is to protect the privacy of our pupils and we do not comment on individuals."