THREE RHYMING RIDDLES

1)
In music I encompass the notes and tones;
In rivers and sea I cover flesh and bones.

2)
Three brothers who work all day and all night,
In close alignment they stand;
Controlling our lives in darkness and light,
They speak not, but we obey their command.
They work without pay in a never-ending cycle,
Without them our lives would be chaotic and frightful.

3)
Called by three letters
To disguise my long name,
In every person,
But never the same.

Answers:

1. SCALES
2. RED, YELLOW, and GREEN - traffic lights 
3. DNA