Look at the attached pic:
looking at point
X (=Vo) :
you need to ask, at what voltage at point X does D1 actually conducts?
The answer is:
1v +1ma*1kohm+0.7=2.7 v
if X is at 2.7v what is the state of D2 and D3 ?
The answer is they are cut-off(not conducting).
Now you ask what is the voltage at Vi
The the answer is 2.7+1ma*3khom=5.7v
If Vi is greater than 5.7v you have :
Vo=(Vi-1-0.7)*1/4 +1+0.7=Vi/4 +1.275
at Vi=10v we get Vo=3.775v
look at the graph
to complete the task you need to look at
D2 and D3 and ask at what voltage do they conduct both and only one ( point Y).