YoLink Smart Freezer Thermometer, LoRa Wireless Fridge Temperature Sensor 3-Pack, Ideal for Refrigerator, Freezer, Greenhouse, Pet Cage, App & SMS Alerts, 5-Year Battery, Hub Included

Fastest and simplest would be to buy 3 Yolink sensors and a hub. Here's everything you'd need for $55. App on your phone can alert you or you can set up emails or texts. The number of texts you get for free is limited (it'd be 25 texts/month with 3 sensors and a hub) but if you set the alert properly that's not a problem. You can name them whatever you like within about a 30? character limit. Battery life is generally great and range is too, so you don't need to worry about the sensors being wholly contained within the freezers.