I am going to visit Thailand in January 2018. I only have 3 Thai numbers and I hope to use a SIM card primarily for navigation and internet browsing (around 6 GB per month). I will be staying in Bangkok for all 3 months.
What is the best package and network provider that suits my requirements?
AIS has plans with unlimited internet that go up to 30 days, with different speeds. AIS probably has the best coverage in Thailand, and is quite reliable. You can top up just about anywhere, including BTS stations.
You can get the SIM card at the airport, and start your first 30-day plan. However be careful, as the staff tends to push useless and/or expensive plans on to the clients. Just look up which plan you want online, and tell them to activate that. Then you can top up and re-apply yourself for the plan 30 days later.