Democratic Unionist leader Rev Ian Paisley welcomed the resignations, saying they dealt a "hammer blow" to the rival UUP.
The North Antrim MP said, "These three MLAs have recognised that their mandate to work for a new and better agreement for Northern Ireland cannot be achieved from within the confines of the UUP.
"The Trimble-led Ulster Unionists are intent on reviving and resuscitating the old, failed Belfast Agreement, an agreement which almost two out of three unionist voters resoundingly rejected last month.
"It is evident that the leadership of the UUP remains wedded to a process that has done nothing but damage the Union and unionism."
Dr Paisley said unionists were beginning to come together to remedy "the failure and deceit" of Mr Trimble's policies.
"The DUP will work with unionists of a like mind who share the DUP's goals of securing a stable and lasting settlement in our Province," he said.
"We look forward to talking to Jeffrey, Arlene and Norah to find a way in which we can improve the unionist position and best move forward and secure a fair deal for the people of Northern Ireland."