1、要.......了means something will happen certainly,but we not sure when it will happen.we use the example from Snoo:
天色昏暗,要下雨了:Under the dark sky, it is going to rain.
看样子要下雨了:there going to have some rain, by the look of it
2、快要......了means something will happen immededately
3、就要......了means somethng will happen soon,the same with 快要...了
example:看样子快要下雨了:it going to rain soon