Stub function calls inside a function

I'm writing a web app using nodejs. I have some utils functions which I export and test separately. There is another function though, that calls some of those utils functions that I would like to stub (using sinon).
From what I've seen so far, I can only stub methods on objects, so how should I design my utilities functions so I can test all of them?

function util1() {
    ...
}

function util2() {
    ...
}

function util3() {
    ...
    util1();
    ...
    util2();
}

export {
    util1,
    util2,
    util3
}

Inside util3 I want to mock (or stub) the calls for util1 and util2.