How do I write three functions to one in comsol?

Excuse me. I want to ask a question. when 0=<t<=1, the function is f(t). When 1<t<=2, the function is g(t). When 2<t<=3, the function is h(t). How can the three functions be written one? I write "f(t)*(t>=0&t<=1)+g(t)*(t>1&t<=2)+h(t)*(t>2&t<=3)", but it is wrong. What shoud I do?

Best your wishes,

Xin Wei