changing note beaming and flags in 6 8 time?

I have a piece that begins, in a partial measure with 4 8ths instead of 6, on beat 3 of a 6/8 measure, and I'd like the flag/beaming to be single 8th, then three beamed together, but I can't make it change from three together (first) and 1 separate (last). Suggestions? Thanks in advance!