Sony: Snake, Spyro, Sir Dan, Crash, Cloud
Nintendo: Mario, DK, Link, Kirby
Microsoft: Master Chief, Ryu Hayabusa, Marcus Fenix, Forza race driver

Nintendo: Mario, Link, Donkey Kong, and Pikachu. (Same as OP)

Sony: Crash, Spyro, Kratos, and Nathan Drake. (Spidey has only had one major exclusive game. While it was amazing. And spectacular. And ultimate. It's not nearly enough to put him on the Mt. Rushmore of Sony. Especially when there are many more names that I would put before his.)

Microsoft: Master Chief... That's pretty much all I got.

Nintendo: Mario, Link, Pikachu, and Samus
MS: Master Chief Helmet, Racer Helmet... and... idk an "X"?
Sony: Nathan Drake, Kratos, LBP dude, also a Racer Helmet dude.

I forget about chicken chaser, Fable is actually a good call. 

Why doesn't anyone mention Luigi, Peach, or Bowser for Ninty? Really, U think Nintys monument would be nothing but Mario Bros. characters.

Nintendo: Mario, Link, Pikachu, Mii.
Sony: Crash Bandicoot, Spyro the Dragon, Kratos, Nathan Drake.
Microsoft: Master Chief, Marcus Fenix, Player Avatar, Cuphead.

Think it needs to be exclusives.

Sony has to ditch Crash and Spiderman, add Ellie (Last of Us) and Aloy (Horizon Zero Dawn)

Microsoft is a bit stuck. Probably needs the Arbiter and erm...  yeah forza car is the best I can think of as well.

Nintendo: Mario, Link, Pikachu, Kirby
Sony: Nathan Drake, Kratos, Sackboy, Ratchet
Microsoft: Master Chief, Marcus Fenix, unnamed Sunset Overdrive protagonist, Jack Joyce

Nintendo was the easiest to pick. Mario and LoZ have been their staple series since the beginning and Pokemon is the biggest thing to ever exist. for a fourth game to represent i thought of Smash Bros but since that game has no mascot, I chose Kirby since he is heavily used to help promote the game, sue to Sakurai being the lead. Sony was easy for the most part, Nathan Drake and Kratos being the most obvious. but then i decided that in order to truly represent the platform, they should be an exclusive, and that threw out a ton of potential icons, leaving me with Ratchet as the most recognizable one. Microsoft was the hardest since most of their games are faceless. Master Chief was the only obvious choice, and Marcus Fenix was a rough second pick. and then just due to having a scarce number of exclusives with notable mascots, Sunset Overdrive guy and Jack Joyce from Quantum Break ended up being the last two

Nintendo: Mario,Pikachu,Donkey Kong,

Sony:Nathan Drake,Kratos,Crash, Solid Snake

Microsoft: Master Chief,Marcus Fenix....then it gets tough. Cortana probably has to be on there since they integrate the cortana name into microsoft in general,and Fable hero

Nintendo: Mario, Link, Donkey Kong, Pikachu.
Sony: Solid Snake, Cloud Strife, Crash, Kratos.
Microsoft: Green Master Chief, Blue Master Chief, Red Master Chief, Yellow Master Chief.