Linux Crontab command is a powerful utility that is used for scheduling and automating tasks in Unix-like operating systems.

In this mission your task is to parse a crontab timing expression (crontab format MIN HOUR DOM MON DOW), return a sorted ascending list of next 3 execution datetimes based on a provided current datetime (ISO format YYYY-MM-DDTHH:MM).

Your script must support all standard crontab expressions:

• Single numbers: 5, 10, 15;
• Ranges: 1-5;
• Steps: */2, 1-10/2;
• Lists: 1,3,5;
• Special character: *;
• Special keywords: @yearly (@annually), @monthly, @weekly, @daily (@midnight), @hourly.

Input: Two strings (str).

Output: List of three strings (str).

Examples:

assert next_crontab_exec("* * * * *", "2025-03-03T14:30") == [
    "2025-03-03T14:31",
    "2025-03-03T14:32",
    "2025-03-03T14:33",
]
assert next_crontab_exec("*/15 * * * *", "2025-03-03T14:30") == [
    "2025-03-03T14:45",
    "2025-03-03T15:00",
    "2025-03-03T15:15",
]
assert next_crontab_exec("5,10,15,20 * * * *", "2025-03-03T14:07") == [
    "2025-03-03T14:10",
    "2025-03-03T14:15",
    "2025-03-03T14:20",
]
assert next_crontab_exec("10-20 * * * *", "2025-03-03T14:05") == [
    "2025-03-03T14:10",
    "2025-03-03T14:11",
    "2025-03-03T14:12",
]