(3) I find it hard to identify any clear difference in meaning between (1) and (2). (3) does not feel right. You can say "I sing when I am walking / when I walk".
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yzh1978I am singing as i am walking.(1)I am singing while I am walking.(2)I am singing when I am walking.(3)I find it hard to identify any clear difference in meaning between (1) and (2).