snipe-it/app/Http/Requests/AssetCheckoutRequest.php

<?php

namespace App\Http\Requests;

use App\Http\Requests\Request;

class AssetCheckoutRequest extends Request
{
    /**
     * Determine if the user is authorized to make this request.
     *
     * @return bool
     */
    public function authorize()
    {
        return true;
    }

    /**
     * Get the validation rules that apply to the request.
     *
     * @return array
     */
    public function rules()
    {
        return [
            "assigned_user"   => 'required_without_all:assigned_asset,assigned_location',
            "assigned_asset"   => 'required_without_all:assigned_user,assigned_location',
            "assigned_location"   => 'required_without_all:assigned_user,assigned_asset',
        ];
    }
}