Eat a banana, carry one 0.5 miles, drop it, and return.  Repeat.
Total bananas: 4

        0       1       2       3
        |---.-:-|-------|-------|
Bananas     2
Camel   C

Eat a banana, carry one 0.75 miles, drop it, and walk back to the 0.5 mile mark
Total bananas: 6

        0       1       2       3
        |---.-:-|-------|-------|
Bananas     2 1
Camel       C

Eat a banana, carry one 0.5 miles and drop it at 1.0, and return to the 0.75 mile mark (with 0.25 bananas left in the stomach)

        0       1       2       3
        |---.-:-|-------|-------|
Bananas       1 1
Camel         C

Pick up the banana, return to the 1 mile mark

        0       1       2       3
        |---.-:-|-------|-------|
Bananas         2
Camel           C

Eat one banana, carry the other one to the 2 mile mark

Eat the last banana, walk to the 3 mile mark, and hope the location of the oasis on your map is correct