how can we combine synchronous function, callback function and promise all together

If I have three types of functions.

1. synchronous function

   function a(){ return "a"; }

2. callback function

   function b(callback){ setTimout(callback(b), ) setTimeout(() => { callback(b); }, 10); }

3. Promise function.

   function c(){ return new Promise.resolve('c') }

How can we combine the result of all 3 in a single promise. like Can we create a function, which can return ['a', 'b', 'c']; I do not want to change these three functions.

how can we combine synchronous function, callback function and promise all together

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