Maths is useful subject in both CSE and ECE as in CSE we need coding which need genral mathmatics and computing is a must and in branch like ECE we have to slove mathmatical equation for implementing an electronic idea . So these all are connected somewhere. 

Maths and computing is more about implementation of aspects of mathematics like linear algebra, engineering calculus. These aspects are currently in extreme use considering machine learning, AI.

ECE is basically study of digital design, semiconductors, gates.

CS includes study of concepts of OOP, Data Structures and Algorithms etc.

All three are interconnected due to coding since modern languages like Python, Ruby on rails are instrumental in all 3.