In my difference-in-difference setting (double diff), I examine the impact of anticorruption laws on firms' asset growth of all countries all over the world after the laws are implemented in each country.

I normally control for firm and industry * year fixed effects in this case following existing literature.

However, I reckon that the impacts of laws will be different between developed and developing countries. Therefore, I am thinking of using subsample tests. There are two ways of conducting a subsample test are1) divide the whole sample into two subsamples and then run the main regression for all subsamples or (2) add the interaction for one subsample and see the difference by reading the interactive coefficients by running the regression.

Regarding the method (2) in double diff, we call it diff-in-diff-in-diff or triple diff. And (2) is preferred compared to (1).

So, what I want to ask is, if in the main specification, I control for firm and industry * year fix effects, so what fixed effects I should control when I perform the triple diff to examine the additional impact of laws on developed countries?