There's a couple missable merchants - Greirat the Thief and Orbeck of Vinheim. Greirat can be recruited fairly early on, but you can send him on missions to expand his inventory a couple of times, but he'll only come back after you've killed a boss so if there's not a lot of bosses left you can miss out on some decent stuff from him. Orbeck of Vinheim requires a minimum level of intellect to recruit (10, which you can get even if you don't start as an intellect-heavy class by equipping a certain ring) and then you need to find and give him one of the magic scrolls to make him stay (there's two in the area near where you find him anyway, just explore around a bit) otherwise he'll leave after you beat the first Lord of Cinder.
Multiplayer is a lot easier in D3, I've found - Embers are a lot more common than humanity (by the 4th boss I've already got something like 30 embers). Need to be embered to be invaded and to summon help, usual deal, but the 30% hp buff you get from using 'em makes them fairly useful if you play solo anyway (plus you heal when you use them, so if you're ever desperate and out of Estus...).
Gameplay-wise it's a little more Bloodborne-y than 1 and 2 - blocking is still useful, but unless you've got a fantastic Greatshield you're better off rolling. Getting carry-weight below 30% is damn near impossible unless you're doing a very specific build, but mid-rolling is good enough to dodge everything that gets thrown at you anyway, and now that armour has gone a Fashion Souls way (wearing 4 pieces of gear gives a flat bonus, plus whatever stats the pieces have) it's not too hard to stay <70%. Can still be fun to slap on Havel's set and dual-wield great shields, that always seems to fuck with people in PvP.
Now, for all achievements, this is where it gets 'fun' - item grinding. The ending achievements are fine, you might need a guide for one of 'em since it's a bit convoluted, but the real problem is the 'get all of x' achievements, since there's a few rings, spells and pyromancies that are locked behind Covenants. This is an issue since to rank up in some of the covenants requires some unintuitive invading and requires a lot of items to rank up (30 items to hit rank 2, which you need for some of the achievement gear); the only option outside all those invasions (where you'll be ganked a lot, because everyone hates the Aldritch Faithful) is to farm rare drops, which can be insanely time consuming depending on the enemy and area. The Darkmoon Blade and Mound-Makers are the worst for this, since they drop from either powerful enemies (Silver Knights for the former) or enemies far from a bonfire (Skeletal Reapers for the latter).
You can get some decent item-find gear at the very least (luck is utterly worthless to level up for this, mind), but whether you do invasions or you do item-farming it's going to take a long time to get all the Trophies. Also, you're better off doing the item grinding in the first playthrough, since the health-scaling of NG+ can screw over fast-farm builds.