You will need to set up an equation. Tools>Equations...
Set an equation where Line1 = 100 - Line2 - Line 3; or something like that.
I typically don't use equations though. I try and use geometric properties and constraints, along with reference geometry, to keep my sketches automated this way. So I would use the "equal length" constraint for each segment of the bellows, along with a dimension for the first line. Then I would create a construction line between the beginning and end of the bellows, and then have a dimension to control this length.
in the general case, you can use equation(s) to get this functionality. in the case of your bellows, it would probably be easier to sett each of the sections to be equal.
Welcome to the forums.
I was working on a part for an example, and agree with the previous posts,
See attached part in SW2009 for an example. I made two configs (Extended and Retracted) with the Over All Length the driving number.
Bellows001.zip 251.8 KB