Singapore-HangZhou on 29 Jun
HangZhou-Beijing on 3 July (evening flight)
Beijing-Singapore on 5 July (evening flight)

Maybe you can try Singapore-Beijing Round trip and Beijing-Hangzhou Round trip tickets since round trip tickets will be much cheaper sometimes. Then your travel plan will be:
Singapore-Beijing on June 29 and then fly to Hangzhou on the same day
Return Beijing from Hangzhou on the evening of July 03 and the fly from Beijing to Singapore on the evening of July 05.

And you need these tickets:
Singapore-Beijing Round trip tickets (depart on June 29 and return on July 05)
Beijing-Hangzhou Round trip tickets (depart on June 29 and return on July 03)
This will meet your travel purpose.

You can consult a booking center about the cost for the two travel schedule respectively. And choose the more economic one.