Philly suburbs doesn't really narrow prices down, since different suburbs around Philadelphia, and the different areas of Philly itself, have demographics that can be anywhere from a large percentage of people living on food stamps, to heirs of estates like Campbells soup. There are also plenty of people in the middle tier as well.
Contractor rates vary, and I'm not sure what a "quality" professional painter, experienced with dealing with older houses would run, but $100 an hour for an experienced professional, and $50 for an apprentice seems usual in some parts of Philly for jobs like plumbing and electrical. $5,500 for three rooms is just over $1,800 per room, or per day if you figure three days. If you estimate the days at 10 hours each for a master painter and assistant, you wind up at $4,500, with $1,000 left over to cover paint, plaster, supplies, and maybe "Oh Smerf" expenses. I wouldn't want to spend this if the guy was just an average american "painter" though. I also wouldn't take references from a single person, unless you were familiar with the before and after work, and or knew the person well for a cery long time.
As far as the work that needs to be done, patching holes in old plaster, like plaster and lathe, can be tricky to do right. Joint compound or patching with plaster board can look like crap, or flake of old lime plaster over time, and even wet plaster can be problematic, and the wet plaster doesn't dry quickly.
I would be somewhat skeptical of someone who says they usually underbid. I've known honest contractors who may mention a job was more difficult or took longer than expected once the job was done or towards the end, but not usually before they've started.