Welcome! You might want to put the post in the Boston area forum.
Here are some thoughts.
PM Questions:
I'm not entirely sure what you're asking here. Three percent is low, but is that what your employer is going to pay you? You are an in-house employee. The company needs to make money to pay for office space, insurance, unemployment insurance, benefits, your bosses, corporate registration fees, etc., etc.
8-12% is what investors expect to pay a third-party property management company to manage units. In addition, large investment groups that self-manage will charge investors approximately 8-12% for property management.
This about Realtor fees. Let's say you are selling a house -- the agent says there is a 5% fee on your $1m property -- that's a lot of money! The agent must be rich. Half goes to the buyer's agent and maybe another half or so goes to the agent's broker. So what looks like 50k in the agents pocket might be $12,500 or less -- of course they then have to pay taxes on that.
PM Tips: You really need to have all of your documentation is in order. If you take security deposits, know the MA security deposit laws like the back of your hand. Personally, I think cash payments are fine so long as you have a receipt -- but I've had one cash payment in over a decade. You really don't want tenants who only operate in cash. Other tips really depend on the type of properties / locations you are managing. Young professionals / recent college grads close to Boston are very different than lifelong renters.
Non PM Questions
There aren't a lot of 4-plexes around Boston. I have one in Cambridge (added the 4th unit) and I think only 2 or 3 4-unit buildings have sold in the last year in Cambridge. As for targeting specific areas, you might want to reach out to @Lien Vuong or another investor friendly agent.
For house hacking you're not going to use other people's money (besides a bank). The structure doesn't work. What would you offer to an investor? At that point you're going to need to pay rent to the investor which basically kills the entire concept of a house hack.
In a city like Boston, notwithstanding COVID, you can even house hack a condo. Buy the largest condo you can afford and fill the bedrooms with tenants. The city is full of fun, interesting people around your age that are looking for a bedroom and roommates because either (a) rents are too expensive and/or (b) they don't want to live alone.