I.
I can separate two high places.
Remove one letter, and I can separate two high places.
II.
I can connect two high places.
Remove one letter, and I can connect two high places.
III.
I can separate two high places.
Remove one letter, and I can connect two high places.
Notes:
- All of the words are singular nouns.
- "High" is relative.
Hint #1 for III:
Both are covered by water.
Hint #2 for III:
I never said you had to remove the first letter.