Not a whole lot you can do if it is formatted as text since you'd have
to control the line up by adding and subtracting spaces. I'd suggest
splitting each cell into three cells: amount, +/- sign, variance or
something like that. You could use a string manipulation function to
parse up the current data.

A1: 3.4 ± 3.7%
B1: =LEFT(A1,FIND("±",A1)-2) or 3.4
C1: ±
D1: =MID(A7,FIND("±",A7)+2,100) or 3.7%

Technically, the result is text but Excel is smart enough to let you
use the result in math operations.

- John