Because you say that it is for general, I won't go into the nitty gritty of teammates and stuff and just provide a brief, overall explanation of each. Also, keep in mind that I have noted Landorus as Incarnate Forme.
Terrakion:
Probably the best of the three that I know of. Terrakion can serve as a huge wallbreaker as a lead and can pack a big punch alongside choice band with its already colossal 129 base attack. Defensively, due to its weak typing, weak to a whopping 7 typings and only 6 resistances isn't especially great. However, with the correct investment, it can still potentially outlive some hits.
I'm not much of an expert in singles, but I am sure that it can efficiently sweep and take out teams that are weak physically. Stealth rocks won't pose much of a threat to Terrakion either - which leads it to become a choice band powerhouse.
However, in doubles... ohohoho.... doubles.... Terrakion + Whimsicott has been in ALOT of teams in this format, starting as leads and causing havoc among teams who don't have a redirector. The plan is to abuse Terrakion's Justified ability by using beat up into Terrakion, which effectively boosts Terrakion's attack by 4 stages - 2 swords dances off in one turn. On the following turn, whimsicott can additionally set up Tailwind to allow Terrakion to be the fastest thing alive. The only counter for this that I can think of off the top of my head is Mimikyu, who would have to set up Trick Room whilst its disguise is broken followed by actually having to take out Terrakion. It is without a doubt that Terrakion would be a dominant force in doubles. However, in singles, I'm not too sure whether or not it is as viable as it is in singles. However, it would serve well in a monotype team due to the fact that there aren't as many fast and hard-hitting rocks like Terrakion.
Landorous:
I wasn't around until gen 8 in Galar. However, I can offer some of the things I note when I take a quick glance at a Pokemon and evaluate it.
Firstly, its x4 weakness to Ice isn't superb. However, due to the lack of ice types in the format and throughout the entire meta over the past few years, I can safely assume that ice types won't pose too much of a threat due to the lack of its usage. The only thing you would have to be careful about is if a Primarina carried an ice beam alongside it.
SINGLES: Sheer Force Landorous (Incarnate) could run some interesting gimmicks such as being a proficient mixed attacker which could surprise the opponent and catch them off guard. Its 115 sp.atk could be usable. Slap a life orb on, alongside sheer force and it can efficiently 2HKO most targets. However, from watching 2017 and 2018 VGC on YouTube, I have seen a lot more Therian Formes than Incarnate. And this would apply for singles too.
Firstly, Intimidate is one of the most cherished abilities of VGC, enforcing some mons to be completely shut down because of it. Next, you would have to see its beastly 145 base attack. Although a speed drop does incur from changing forms, with correct investment, it could potentially be a 2HKO or 3HKO, and rarely a OHKO unless an Ice beam had found its way into Landorous. One minor issue with Landorous is that it is incapable of using a STAB flying type move, unless you want to try Flynium Z (is that how you spell it?) alongside fly to deal a 1-time massive damage amount. However, Smogon recommends what I had just said above, to use a Flynium Z and substitute + swords dance to turn Landorus into a complete, diabolical monster. It can set up on a passive mon such as Toxapex and completely dominate after getting off 1 or 2 swords dances.
DOUBLES: In doubles, I am sure Therian Forme is superbly better than Incarnate due to the ability, Intimidate. It can reliably check and substitute Incineroar, another Intimidate mon that is widely used even until this day in VGC. Smogon recommends using choice scarf alongside U-turn to effectively pivot around, allowing it to become a nimble Intimidate-Pivot who can bring momentum into the match. Most of its golden points are already listed above too.
Latios: Sigh, we going even further back now
As you mentioned, Latois is the fastest of the three and the hardest hitter. However, like you said, it isn't overwhelmingly powerful in comparison to Terrakion. It has 6 weaknesses, 6 resistances and 1 immunity, not what you would call optimal or the best. However, it does serve as a great special-all-out-attacker alongside a choice specs. Latios gains a mega evolution which I'm pretty sure we can agree that it will automatically be the best of the three. For that reason, I will only cover standard Latios.
Singles: Latios has reliable recovery from recover and can dish out gnarly damage with its stab Psychic and Draco Meteor, alongside hidden power Fire or Surf for coverage. It is also recommended that you use Life orb as to allow it to have versatility in its moves. However, it can effectively be a choice specs killer, similar to choice band Terrakion except special. You mention that it has a base 110 sp.def, something which is definitely decent to have, but wouldn't necessarily qualify as 'bulkies of the three'. When you look at bulk, typing, defense, HP and sp.def is taken into account. Stonejourner has massive defense, but a crippling special defense which could get KOd at the touch of a water gun (don't judge me on that, I didn't check). Latios can wall two or maybe even 3 special attacks. However, its defense is a mere 80, and without intimidate, functions extremely poorly defensively. Its HP is just as underwhelming, being another base 80. This could be slightly similar to Alakazam, who has freakishly high special defense but would still most likely get KOd by a physical move. My point is that just because it has high special defense, doesn't mean it is bulky. It's Bulky specially, but a mere Darmanitan (Galar) could overkill it with Icicle Crash.
Doubles: Nothing really changes. It serves as a hard special hitter and maybe a fast pivot, though not necessarily due to its middling-above average speed.
Conclusion:
Honestly, I would pick Landorous as the best of the three. However, Latios is the best special attacker, Terrakion is the best doubles-gimmick user/choice band hitter and Landorous can be both a mixed attacker and U-turn Intimidate pivot. The bulkiest one would probably go to Landorous whilst the hardest hitter also goes to Landorous. Nonetheless however, Terrakion is another great choice if you don't want dual Flyings and can facilitate extremely well if you are already using Whimsicott in doubles.
If you're talking about in-game, yeah, just go with Landorous because of its superior stats and great typing. Just don't make it go up against ice tho...
