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).