All of these 3 providers provide 2G, 3G, and 4G networks.
In the outer islands of Agalega and Rodrigues, only My.t and Emtel provide a 4G network respectively.
Where to buy sim cards
At MRU airport in Plaine Magnien you can find sim cards at the several car rental kiosks just after exiting the airport. You can also get sim cards at the post office desk at the airport.
For emtel you can use their store locator to find stores to purchase sim cards.
You can also find Emtel sim cards at most convenience stores all over Mauritius. I don't recommend purchasing sim cards here, you'll have to mail your registration and passport info to emtel stores for your sim card to get activated. Your sim card can't get activated at random convenience stores.
Sim card cost: about MUR 100 inclusive of VAT
What you need to purchase your sim card: passport and unlocked phone.
The activation of sim cards is not immediate. For emtel sim cards activation takes about an hour. For My.t sim cards activation can take up to 2 to 3 days.
Orange by the widest coverage. Their prepaid sim card costs MUR 135 and comes with 250 MB, unlimited Facebook, and 150 domestic SMS. This preloaded data package will become active in 2-3days.
Emtel is the best sim card for people with heavy data needs.
They have a data-only sim card (Airbox prepaid) that costs MUR 1499. On this sim card, you can add 5GB, 75GB, 100 GB, or 500 GB data packs. All data packs on Airbox prepaid are valid for up to 30 days.
Emtel recommends a special tourist sim card for international tourists. These sim cards cost MUR 200 and come with 1GB data valid for 10 days, unlimited Facebook, and 100 domestic SMS.
Compared to emtel and orange, Chilli has lower rates though it also has the poorest coverage on all islands.