Three are not enough seriously, I struggle everytime because I don't even have an absolute favorite, I'll just list favs for each gen:
>I
Persian, Tentacruel, Magneton, Omastar, Magneton, Beedrill, Articuno, Weezing, Scyther, Dragonair, Graveler, Seadra, Kadabra, Porygon, Dewgong, Venomoth, Slowpoke, Slowbro, Pidgeotto, Dodrio, Ninetales, Oddish, Paras, Shelder, Cloyster, Exeggcute, Exeggutor, Krabby, Kingler, Weepinbell, Kabutops, Wartortle Ivysaur, Zapdos, Sandshrew, Sandslash
>II
Skarmory, Suicune, Kingdra, Espeon, Lugia, Umbreon, Porygon2, Qwilfish, Slugma, Magcago, Steelix, Forretress, Bellossom, Larvitar, Corsola, Scizor, Totodile
>III
Tropius, Relicanth, Skitty, Lunatone, Solrock, Registeel, Regice, Regirock, Ninjask, Masquerain, Claydol, Torkoal, Spheal, Sealeo, Beldum, Metagross, Absol, Chimecho, Latios, Deoxys Defense
>IV
Budew, Spiritomb, Bronzong, Leafeon, Porygon-Z, Grotle, Torterra, Regigigas, Snover, Probopass, Shaymin Land Form, Cherrim, Skorupi, Arceus, Giratina Origin, Vespiquen, Carnivine
>V
Volcarona, Virizion, Stunfisk, Joltik, Karrablast, Sandile, Ferrothorn, Sigilyph, Whimsicott, Vullaby, Zorua, Darumaka, Liepard, Lampent, Chandelure
>VI
Carbink, Pumpkaboo, Trevenant, Dedenne, Doublade, Espurr, Volcanion, Mrga Abomasnow, Mega Manectric, Mega Scizor, Mega Venusaur, Mega Charizard Y, Mega Sceptile, Mega Gyarados, Mega Banette, Mega Steelix, Greninja, Mega Audino, Spritzee, Aromatisse, Primal Kyogre, Primal Groudon
>VII
Dhelmise, Minior, Rowlet, Torracat, Nihilego, Celesteela, Kartana, Solgaleo, Marshadow, Necrozma Dusk Mane, Alolan Marowak, Cutiefly, Oricorio Forms, Passimian, Zeraora, Alolan Vulpix, Alolan Ninetales, Alolan Sandshrew, Tsareena
I think...
