I've never seen them in my garden until shortly after I got a mint from a friend. The one right beside mint is smaller.
Looks like Oxalis triangularis. Am (quite!) familiar with yellow wood sorrel (Oxalis stricta) as an enthusiastic volunteer in my yard...but your plant I know as a pot resident. ? Perhaps it was cohabiting with the mint.
It's been a while. Yes it was false shamrock (love plant) which was in a pot and thrown away thought dead. I found out this thing could actually over winter here in BC. It would die back in winter and is coming back right now.
