"增加" ：Increase but with some meaning of add and plus. For example , 增加budget
"增长" : 2 meanings , depend on situations and with different pronunciation . 1: Increase length . 2.Increase things which can't be measured precisely. (增长wisdom / bacteria增长 )
"扩大" : Extend size/amount.... For instance : 扩大demand(economy) , 扩大space.