What you will have to learn to succeed eventually is to have a look at your bases entire demand and select 2-3 types that fit it well. Also remember demand will grow a bit over time, so rather pick a bit oversized than a perfect fit.
Many new players make the mistake to chose their aircraft after single routes, which consequently ends in them having a suboptimal fleet choice. I noticed this with your earlier try in Boston, you acquired the 747, which makes no sense in Boston, because it can only fly to a few destinations from there. So to get the other destinations served, you needed another type. In cases like that, rather take the smaller one and fly twice a day to the destinations you would use a 747 to. Generally, multiple long haul fleets only make the least bit of sense in the worlds top 10 airports. And even there I would mostly go with a single long haul fleet. LHR is the only base where two differently sized long haul fleets make real sense.
For short haul, it is okay to go over the 3 fleet threshold in the expansion phase if you can't get enough of your main type. Just make sure you don't get so many that replacing will become a real problem.
So, the perfect fleet choice suits the MAJORITY of your potential routes. If it's too big for some, just pass on them. That's okay. If its too small - just fly multiple times. Getting a different sized aircraft for every route size is maybe THE most frequent newcomer mistake I see out there.