To do high key photography, generally the background is two stops brighter than your main light.
Also a 3:1 lighting ratio on the subject is having the fill, one stop less than the main light. Since you are using a reflector as fill, you kind of throw out the equation. (Usually you would have the main at say f11 and the fill at f8 no reflector)
If you use a reflector, just meter the main, then the shadow and feather the lights to get the 3:1 or 5:1 ratio.
In your situation with only three lights, with high key, I might try two lights, opposite sides equal distance angled in at the background, having the background reading evenly f16. Then light the subject with one light and fill with the reflector. Set the main for f8.
If you are doing only once person you may get away with one background light but you may still get falloff and an uneven background.
Personally I find a silver reflector to be too harsh and would use white fomecore or a white reflector.
Also feather your mainlight. Than means place the hottest spot of the light at the farthest portion of the face. This gives a more wraparound effect and also places more light on the reflector which makes the fill closer to the amount of light as the mainlight.
In high key photography usually you don't want very much contrast so rarely more than a 3:1.
The umbrella is fine behind you. The umbrella will easily wrap the light around you and hit the subject. It is not a good idea to use a long remote cable release because if you want to interact with the subject you can't see the results away from the camera.
Also what I do is meter the main light and with a reflector fill set the camera to about half or a full stop less than the main which will still give you shadow detail. The fill is not supposed to do anything more than fill the shadows and the shadow side of the face. Your lighting ratios 3:1, 5:1 7:1 will give you the differing amount contrasts. So concentrate more on you main light than anything. The fill just controls the contrast.
If this is confusing personal mail me.