Paint the visible vertices with auto normalize active
--the brush should be set to 0.100 weight 1 strength and Mix.

Bottom's resulting weights from highest to lowest:
0.100
0.100
0.100
1.000

Middle's resulting weights:
0.540
0.810
0.900
0.000

Top's resulting weights:
0.360
0.090
0.000
0.000