@pinkpink What kind of thermos are you using? They have nice stainless steel ones that keep something hot or cold for seven hours or so. You might want to invest in one if you keep making your shakes.
Three hours doesn't seem long, but if you keep it cold, I don't see why it would be a problem drinking it.
Welcome to the forums.
I pack up all my food for the day in a cooler and have done so for 3 years. I have had no issues. I do make sure i add an ice pack