Oh no, sorry if I miscommunicated with you. The other two mushrooms only come out later this year! It's not currently available to play.
The latest build of the game that I updated this week is just an update to Button's route (containing the new bedroom background, main menu, and more graphics like kissing/death scenes, and 2 more meals you cook her into).
The gallery folder from the main menu should have 4 total art pieces for Button, and if you got all her endings, they SHOULD be all there. Maybe you've found an error in the game? Let me know which ones you're missing, eg. "top left".