With the Holiday 2017 event well underway and more gen 3 pokémon being released, I compiled some stats on how common the current gen 3 and Delibird spawns are. Please note that with the current event, gen 3 starters are spawning more frequently than usual.
Very Common
Delibird, Gulpin, Roselia, Spheal, Wurmple, Zigzagoon
Common
Electrike, Makuhita, Meditite, Mudkip, Plusle, Poochyena, Seedot, Shuppet, Snorunt, Skitty, Spoink, Torchic
Uncommon
Ralts, Treecko, Zangoose
Rare
Barboach, Corphish, Delcatty, Duskull, Hariyama, Linoone, Mightyena, Sableye, Shroomish, Slakoth, Wailmer
Very Rare
Banette, Carvanha, Feebas, Grumpig, Luvdisc, Marshtomp, Nuzleaf, Sealeo, Swalot
Exceedingly Rare
Breloom, Combusken, Crawdaunt, Dusclops, Glalie, Grovyle, Kirlia, Lombre, Lotad, Sharpedo, Vigoroth, Whiscash
A reminder that 2nd stage evolutions for gen 3 mons do not spawn in the wild. These include Beautifly, Blaziken, Dustox, Gardevoir, Ludicolo, Sceptile, Shiftry, Slaking, Swampert, and Walrein. There has also been no reports of Absol, Cascoon, Mawile, Medicham, Milotic, or Silcoon sightings yet in Winnipeg.
These spawn frequencies do not take into account any weather boosts, since the data was collected during points of extreme weather (where no pokémon had any boosts). Any weather boosts may shift the affected pokémon up one tier.