Very Simple Triangles
Pyth, 44 42

ItQpdd*\_*4/Q2)jbms&lt;*dQhQ,c" /\ "2,\/"__\\


The first line:

ItQpdd*\_*4/Q2)
ItQ           )    If the input is not 1
   pdd             Print two spaces
      *\_*4/Q2     Then groups of 4 underscores, repeated input/2 times.


The other two lines are generated by noticing that the second line consists of " /" and "\ " alternating input + 1 times, and the third line consists of "/" and "__\" alternated in the same fashion.