Yeah I remember doing this as well. I used to play a lot of competitive singles back in gens 5-6 (when I didn't have a job) so instead of giving you exact pokemon I'll give some general things anyone reading this will want
1. Since singles is only 3 pokemon, hazards (aside from sticky webs) aren't really that big of a deal. If you're using a pokemon with a 4x weakness to rock (most likely charizard or volcarona,) then I highly recommend adding a defogger/rapid spinner to your team.
2. Every pokemon should have a different role. Since singles is with only 3 pokemon, the roles I suggest having are
a. offensive lead. Usually this mon is responsible for hazards but since hazards aren't as big of a deal with 3 pokemon I suggest a decently bulky pokemon that hits hard and that can easily pivot into your other 2 mons. Examples include volt switch Rotom (recommend wash since heat is gonna take 1/4 dmg to stealth rock) and u-turn gliscor
b. wall. Tanky, reliable, and can easy to switch into. Think of this as the pillar to your team. Usually walls will stall with toxic damage and can heal themselves. Examples include ferrothorn, regenerator toxapex, unaware quagsire/clefable, amoongus, and eviolite chansey.
c. cleanup. Fast, powerful, and usually bad defenses. Cleanup mons outspeed partially damaged pokemon and finish them off with ease. Recommended that this pokemon uses the attack type your lead doesn't (i.e special attacker if your lead is a physical attacker.) Examples include speed boost blaziken, moxie gyarados, and excadril.
Additional info: If possible make sure your team doesn't have 2 pokemon weak to the same type. Light clay with a screen setter such as alolan ninetails or prankster meowstic will also be really nice, especially if you want to sweep with a single mon (e.g dragon dance gyarados or swords dance excadrill.) Give items that work well with pokemon, and don't ignore the fact that you have access to every single weakness berry.
If you want to know what I ran to win though here's what I ran (I did not breed for IVs)
1. Togekiss (Defensive Lead)
Item: Light Clay
Ability: Serene Grace
Nature: Timid
EVs: 252 Spe 252 HP 6 SpA
Light Screen
Reflect
Air Slash
Wish
2. Quagsire (Wall for setup mons)
Item: Leftovers
Ability: Unaware
Nature: Careful
EVs: 252 Sp De 252 HP 6 De
Curse
Earthquake
Toxic
Recover
3. Blaziken (Cleanup)
Item: Focus Sash
Ability: Speed Boost
Nature: Adamant
EVs: 252 Spe 252 Atk 6 HP
Protect
High-Jump Kick
Flare Blitz
Swords Dance
If you want to copy that by all means do so. All you really need to do is set up screens with togekiss, wish, switch into blaziken/quagsire, and usually you'll be able to win from there. If you are having trouble with the... last battle, then sub out any of these mons for something like aegislash or weavile. Good luck!
