sum_triplets_interval1.py

def sum_triplets_interval( iList ) :
    # here i will act as as start index and i+3 as end index because  i : i+ 3 offset will include ith, (i+1)th ,(i+2 )th values.Intitially i is 0.

    res = [ ]
    for i in range( len( iList ) ) :res.append( sum( iList[i : i+3 ] ) )

    return res

print sum_triplets_interval( [ 10 , 20 , 30 , 40 ,50 ] )