Arriving at the airport ninety minutes ahead of the scheduled departure would be comfortable. At the very peak of high season I might stretch that to 2 hours. Three hours is unnecessarily early for a domestic flight any time of the year, IMO.

There's a large food court upstairs (pre-security) and many options airside too. There might be some lounges you can access but I've never done it despite going in/out of DMK frequently for domestic flights.

Grab is a perfectly fine way to make it from Lumpini Park to DMK. Figure about a 30 minute ride, give or take. (Typically a little less but you may encounter traffic on the surface roads leading to the highway.)