The National Union of Teachers (NUT) five-day annual conference typically attracts 1,350 delegates.
Brighton will host the NUT event in 2014 and 2016, and Harrogate will host its 2015 conference.
Mammoth Events director George Thompson stated: "The NUT has never previously entered into a contract situation, choosing to tender every year. For them to make this commitment with us is testament to the relationship we’ve developed."
NUT principal officer Karen Handley added: "Mammoth delivered a great event for us in 2013 and we’ve been impressed by their creativity and innovative solutions. Moving forward they presented a compelling case for us to work with them over the next three years."