How triac acts as a diode and two transistor?
Triacs are nothing like two transistors and a diode. Read the Wikipedia article or (better, I think) the GE SCR manual ca. year 197x which will explain things better than anyone here is likely to spend the time doing. 

There are four possible quadrants of operation. The commonly shown two-SCR "equivalent" (which would be like four transistors) is also very misleading (it fails to explain how Q2 and Q4 work at all (reversed gate polarity from MT2 polarity), even if you correct the SCR to a complementary type). 

Look at the layer structure and read the explanations to understand. Some things (like the transistor as two diodes) cannot be even roughly understood by breaking them down into smaller parts (from the GE manual above-linked):