You can use the following code to get what you are looking for.

for (let i = 1; i < 101; i++) {
    if(i % 3 == 0) {
       console.log('THREE');
    } else {
       console.log(i)
    }

}

If you need to write the values to a document, change the console.log to document.write

Put the THREE in some inline element and add css rule to change the color. For printing the list the solution explained by Jack. (Did it differently because I could.)

const text = (new Array(100))
	.fill('')
	.map((_v, i) => (i % 3) === 0 ? `<b>THREE</b>` : i)
	.join('<br/>');

document.write(`<p>${text}</p>`)

b {
  color: red;
}

First, loop through the numbers 1 to 100:

for (var i = 1; i <= 100; i++) {
    //Stuff will go here
}

Then, write the number i to HTML:

document.write(i);

Finally, add the if statement:

if (i % 3) {
    document.write(i);
} else {
    document.write("THREE");
}

Full code:

for (var i = 1; i <= 100; i++) {
  if (i % 3) {
    document.write(i + "<br>");
  } else {
    document.write("THREE<br>");
  }
}

EDIT

Here's how you'd make THREE red:

for (var i = 1; i <= 100; i++) {
  if (i % 3) {
    document.write(i + "<br>");
  } else {
    document.write("<span style='color: red;'>THREE</span><br>");
  }
}