As we know, one of the simplest way to do a summation (with a special intervals) over list elements is using of

Sum[list[[i]], {i, 3, 8, 2 }] (*with steps of 2*)

for a list such as list={a,b,c,d,e,f,g,h,i,j,k,l} which results to c+e+g, how can we use of Replacelist or FoldList to this aim