Three verbs with um, three different meanings.
Well, it's all about circular movements: 90 (falling down), 180 (changing direction), and 360 (moving around) degrees.
The latter differs from the others:
um in umfliegen is not separable, and the accent therefore lays on the verb.
The other two are separable instead, and bear the accent.
For example, if I say "Ich umfahre den Mann", I am saying that I am driving around him.
"Ich fahre den Mann um" instead means that I am knocking him down
by car or bicycle .

The word

Umwelt

employs um in the sense of 360 degrees.
Die Umwelt simply is the world around us, i.e. the environment.
Heidegger uses the word in a rather special meaning, avoiding every assumption about "real beings".
Usually we assume Umwelt is something Räumliches, something spatial, but the philosopher has to put this idea aside.
Die Umsicht again derives from the idea of circularity:
look around (Sicht comes from sehen). Usually we ask others to be umsichtig, we mean they should be cautious, look attentively around.