SOURCE

const form = document.querySelector("form");
const countElem = document.querySelector("input[name=count]");
const printBtn = document.querySelector("#print");

function calculate(a, b, operator) {
    if (operator === "+") {
    return a + b;
    } else if (operator === "-") {
    return a - b;
    }
}

function randomIntInRange(min, max) {
    return Math.floor(Math.random() * (max - min + 1) + min);
}

function randomOperator(operators) {
    return operators[Math.floor(Math.random() * operators.length)];
}

const resolvers = {
    /**
     * 100以内3个数的连加
     * 
     * 比如
     * 
     * 15 + 33 + 21 =
     * 17 + 26 + 33 = 
     * 
     * 不会出现加法超过 100 的情况。
     */
    problem_100_abc_add() {
    const self = resolvers.problem_100_abc_add;
    const a = randomIntInRange(10, 99);
    const b = randomIntInRange(10, 99);
    const c = randomIntInRange(10, 99);
    
    const result1 = calculate(a, b, "+");
    if (result1 > 100) {
        return self();
    }

    const result2 = calculate(result1, c, "+");
    if (result2 > 100) {
        return self();
    }

    return [a, "+", b, "+", c, "="].join(" ");
    },

    /**
     * 100以内3个数的连减
     * 
     * 比如
     * 
     * 78 - 12 - 21 =
     * 63 - 26 - 17 = 
     * 
     * 不会出现减法得数小于 0 的情况。
     */
    problem_100_abc_sub() {
    const self = resolvers.problem_100_abc_sub;
    const a = randomIntInRange(10, 99);
    const b = randomIntInRange(10, 99);
    const c = randomIntInRange(10, 99);
    
    const result1 = calculate(a, b, "-");
    if (result1 < 0) {
        return self();
    }

    const result2 = calculate(result1, c, "-");
    if (result2 < 0) {
        return self();
    }

    return [a, "-", b, "-", c, "="].join(" ");
    },

    /**
     * 100以内3个数的加减混合运算
     * 
     * 比如
     * 
     * 70 - 20 + 12 =
     * 98 - 12 - 13 = 
     * 15 + 33 - 21 =
     * 17 + 26 + 33 = 
     * 
     * 不会出现减法得出小于 0 或者加法得数大于 100 的情况。
     */
    problem_100_abc_add_or_sub() {
    const self = resolvers.problem_100_abc_add_or_sub;
    const a = randomIntInRange(10, 99);
    const b = randomIntInRange(10, 99);
    const c = randomIntInRange(10, 99);

    const op1 = randomOperator(["+", "-"]);        
    const result1 = calculate(a, b, op1);
    if (result1 < 0 || result1 > 100) {
        return self();
    }

    const op2 = randomOperator(["+", "-"]);
    const result2 = calculate(result1, c, op2);
    if (result2 < 0 || result2 > 100) {
        return self();
    }

    return [a, op1, b, op2, c, "="].join(" ");
    },
};

function generateHomework(e) {
    e.preventDefault();

    printBtn.disabled = true;
    const formData = new FormData(this);
    const type = formData.get("type");
    const count = parseInt(formData.get("count"));

    let problems = new Set();
    while (problems.size < count) {
    problems.add(resolvers[type]());
    }
    problems = Array.from(problems);
    let rows = "";
    while (problems.length > 0) {
    const rowProblems = problems.splice(0, 3);
    
    const tds = rowProblems.map(problem => `<td>${problem}</td>`);
    rows += `<tr>${tds.join("")}</tr>`;
    }

    document.querySelector("tbody").innerHTML = rows;
    printBtn.disabled = false;
}

function showCountLabel() {
    const countLabel = document.querySelector("#count");
    countLabel.textContent = this.value;
}

form.addEventListener("submit", generateHomework);
countElem.addEventListener("input", showCountLabel);
printBtn.addEventListener("click", () => print());

<header class="no-print">
	<form>
        <h1>小学二年级数学题随机生成器</h1>
		<div>
			<h3>题型：</h3>
			<label>
        <input
        type="radio"
        name="type"
        value="problem_100_abc_add"
        checked
        />
        <span title="比如
        
15 + 33 + 21 =
17 + 26 + 33 = 
        
不会出现加得数大于 100 的情况">100以内3个数的连加</span>
    </label>

    <label>
        <input
        type="radio"
        name="type"
        value="problem_100_abc_sub"
        />
        <span title="比如
        
78 - 12 - 21 =
63 - 26 - 17 = 
        
不会出现减法得数小于 0 的情况">100以内3个数的连减</span>
    </label>

    <label>
        <input
        type="radio"
        name="type"
        value="problem_100_abc_add_or_sub"
        />
        <span title="比如
        
70 - 20 + 12 =
98 - 12 - 13 = 
15 + 33 - 21 =
17 + 26 + 33 = 
        
不会出现减法得出小于 0 或者加法得数大于 100 的情况。">100以内3个数的加减混合运算</span>
    </label>
    </div>

    <div>
    <h3>题量：</h3>
    <input
        type="range"
        name="count"
        min="6"
        max="60"
        step="6"
        value="60"
    />
    <output id="count">60</output>
    </div>
    <div>
    <button type="submit">生成练习题</button>
    <button type="button" id="print" disabled>打印练习题</button>
    </div>
</form>
</header>
<main>
    <table
        border="0"
        width="100%"
    >
        <tbody>

        </tbody>
    </table>
</main>

@media print {
.no-print {
    display: none !important;
    }
}

form > div { margin-bottom: 1em; }
form button { font-size: 1.5em; margin-right: 1em;}
form label { display: block; margin-bottom: 0.5em; }

table {
    font-size: 20px;
    font-family: Courier, monospace;
}

td {
    padding-top: 0.3em;
    padding-bottom: 0.3em;
}

tr:nth-of-type(2n) td {
    padding-bottom: 1.6em;
}