01. Smallest of Three Numbers

function solve (firstNum, secondNum, thirdNum) {

    if ( firstNum < secondNum && firstNum < thirdNum){
        console.log(firstNum);
    } else if(secondNum < firstNum && secondNum < thirdNum) {
        console.log(secondNum);
    } else {
        console.log(thirdNum);
    }
}

Или с Math.min():

function solve (firstNum, secondNum, thirdNum) {

    console.log(Math.min(firstNum, secondNum, thirdNum));
}