if you allow 6mm per chainring offset, then you could move the chainline from 47.5mm for the middle ring to 47.5mm between the inner and middle (i.e. middle at 50mm) by adding 3mm to each side of the axle, or 6mm overall.
UN54 BBs are cheap emough to buy a couple to try and play with the chainline or you can get a phil woods BB that can be adjusted by 2mm either way which would do much the same.
http://www.hubjub.co.uk/philwood/philwood.htm
However, for double and bash most people dont bother moving the chainline. In fact most people on double and bash are using Outboard bearing BB cups which dont allow for much chainline adjustment anyway.
Id go with a stock BB if I were you and only if you ‘re having trouble get a 6mm longer axle UN54.