It is most definately not ok to collect data in a third group and not report it because it looks better if you don't report it.
There is no question about that.
See for instance this link:
https://replicationindex.wordpress.com/ ... practices/
Also, don't just look at frequencies but run some actual tests, for instance, run a repeated measures anova and see whether your 3 groups differ from pre- to post-test.
I just wanted to edit this message to make it abundantly clear that it is absolutely not ok to do this. This is what's wrong with psychology, according to many people. Go to any psychological conference and you will probably find on or more talks (or at least mentionings) of questionable research practices
. It is a very important point, and I cannot stress enough how much it is not ok to leave out a condition, with no mention of it in a paper, just because it looks good. It is not only questionable science, it also goes against ethics, it is morally wrong, especially (but not only!) when you research interventions!!!!