JSFiddle - React, Tailwind, and code Playground

JavaScript

function sum(number) {
	let arr = [];
  let x3 = 3;
  let x5 = 5;
  while(x3 < number || x5 < number) {
  	if (x3 < x5) {
    	arr.push(x3);
      x3 += 3;
    } else {
    	arr.push(x5);
      if (x3 === x5) {
      	x3 += 3;
      }
      x5 += 5;
    }
  }
  return arr;
}

console.log(sum(30));