One source field to multiple dest fields - how to?

Hi,

I have a source field which must be used to calc values for three dest fields.
For this i was planning to make a tamper plugin, which would do all the logic.
I intended to make 3 mappings of the same source field (which is smart sku,
having some info encoded in it), then in tamper use different custom plugins to get
corresponding values for each dest field. But for some odd reason Feeds Tamper GROUPS
the different mappings by the source field, making it impossible to cofigure each mapping independingly. Why and how to reuse same field more then once?
Another and better approach would be having possibility to feed several dest fields in one plugin. Any ideas how