The development will boast a larger gym, a new studio, nine new changing facilities and staff offices, as well as a new reception and café area.
The swimming pool will also be redecorated, and the main phase of the building work will start in April.
The swimming pool will also be redecorated, and all work on the centre is scheduled to be finished by September when students return after the school holidays.
Lloyds Bank are supporting Ashville with the project.
Ashville College Headteacher Mark Lauder said: “At £3m, this is the single biggest investment undertaken at Ashville College so far.
“The sports centre has been a central feature of the campus for more than half a century, but the decades of constant use by pupils, club members and outside organisations, have taken their toll.
“Once completed, which all being well should be late August, we will have one of the most modern sports centres in the region.
“To lessen disruption, and to reduce the construction timescale, a large part of the new-look sports centre is being made off-site.
“During the Easter holidays these units will be transported to Harrogate and simply lowered into place. This allows Ashville’s games programme to run without disruption throughout the year.
“We are grateful for the support of Lloyds Bank in this venture.”
Joanne Lampkowski, education relationship manager at Lloyds Bank, said: “The new sports facilities will be a great asset to both the college and the wider community, and we expect it will encourage the younger generations to participate in more sporting activities."