Project Euler #9 implementation in Java
There isn't much code, so that leaves little to be reviewed. I have not considered the mathematical side of this. This is also a code stub, so when reviewed out of context it is hard to do a good review. 


Comment it – As a code stub it is hard to understand what you are trying to achieve, and as such I would problably leave a comment stating what you actually wanted to achieve. Maybe just a reference to "Special Pythagorean triplet, Euler Problem 9", or similar. And possibly some of the justification of your formulas
Do standard indentation – The class and the method is at the same level, which is confusing. Do indent, but keep it a standard width, like 2 or 4 spaces. The same applies for how many lines you use between statements, ending braces, methods. Keep it clean(er)
Keep computation in loop conditions to a minimum – In your case it will most likely be optimised by the compiler, but having computational stuff in the loop conditions, like (num/3 - 1), should be avoided. Do the calculation once in front of the loop, and make it a simple comparision in the loop condition
Take care when using parenthesis – You have some unneccessary parenthesis, and some which is kind of strange, I would have written it: b = num - (500*num) / (num-a) which emphasizes that the denominator and the dividend are calculated before the actual division
Comparing doubles and int is dangerous – In most languages a double variable is usually not checked directly for equality as the imprecision in the storage format could leave to interesting effects. See Spot the defect: rounding, part two. However this might not apply to you as you are mainly interested in a whole number and the decimal parts of it...
Variable naming – Nothing indicates that num is a constant, neither in the naming nor in the declaration (i.e. you could use final). And what does num mean? Is it a number, is it a constant?
Consider using a function – In the problem description it indicates that for other numbers you could find other pythagorean triples, which could be extended into making a method like findPythagoreanTriplet(number) which could either return the first triplet, or a list of triplets. Using a function would allow for extending the code stub into maybe doing something more useful like finding other triplets matching some criteria, or similar
Computation vs presentation – An alternative view on creating a function, is that you could then split the computation from the presentation, which usually is a good thing to do. This also according to the principle of single concern for a method
Jammed if conditions – The last few days I've seen a lot of if(someCondition){, which in my view would like nicer as if (someCondition) {. Having a little space here and there increases readability, asOpposedToNotHavingSpaces...


Do be aware that this code review is making some assumptions, and might be a little overkill for something which you might just have thrown together to solve an immediate problem you had. However, becoming a robust and good programmer, starts by doing it right from the smallest through to the largest projects.