I can't speak for the developer, but from my use of this app, I would say that this app doesn't do what you want.
I hope you tried the Go Figure Lite first to discover that this app isn't suited for you.
Let me just say in defense of this app that for me and my needs, it is the best one on the App store. (I don't have a jail-broken phone.)
To accomplish what you want you would first have to execute a set of logic tests to identify which variable is missing. For example enter 0 when this is the variable that you want to solve for and enter a non-zero value for the other two variables.
I might use
Flag 1: And(x = 0, a <> 0, b <> 0)
Flag 2: And(x <> 0, a = 0, b <> 0)
Flag 3 : And(x <> 0, a <> 0, b = 0)
Then I would write out the formula for case 1 multiplied by flag1, plus the formula for case 2 mulitplied by flag2 plus the formula for case 3 multiplied by case 3. So one final result line should solve your equation taking into account the value you which to solve for based on the logic testing provided by the flags.
I hope this isn't too confusing.
And if this doesn't provide you what you want, then either abandon Go Figure or wait for an update that offers this capability. But since I am not the developer, I can't tell you whether this capability is even feasible, although as noted above you can "write" a solution that ought to work for you without having to reformat the formula multiple times. Whether this is a "stupid" solution is your call.