Followup on parallel diode circuits
If the diodes are ideal -


If only D1 exists then V = +3 V because current will flow through R1 and D1 to the +3 V point.
If we add in D2 then, since V > +2 V current will flow through D2 so V will drop to +2 V.
If we now add in D1 then, since V > +1 V current will flow through D1 so V will drop to +1 V.



  I tried to understand this by guessing the answer to be 2 V, and then the top two diodes would be reverse biased, 


That might be a good starting point but soon you realise that if V = +2 V and D3 is connected to +1 V then it is forward biases, will conduct and pull V down to +1 V.