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I was thinking I would place the 2 umbrellas at about eye level, a first one 30 degrees to my right, the other 30 degrees to my left, both shooting trough the umbrellas towards the team, at equal power. I could also use my 3rd strobe directly behind me at higher level, pointing slightly downwards towards the team, at lower power.
Eye level horizontally crossed lights is a recipe for one flash canceling the other resulting it flat light. The back rim lighting is a good idea to create separation but with a group it can also create highlights and shadows in odd looking places.
If you want more natural lighting take your clues from nature where light comes from overhead at a downward angle of 45° to reach past the brow into the eyes and model the features face.
For groups I like to use a butterfly pattern, stacking the lights vertically over / under the lens with fill about chin level and key higher than the faces. It creates the same flattering natural modeling on all the faces. See this tutorial for other tips on shooting groups: http://photo.nova.org/Groups/