Going back to USA after staying there for 3 months recently, problems?

I don’t know what’s wisest but offer opinions because that may be the best available given that your question is about the opinion of an Immigration official:

i) Getting caught in a lie, however “small” and “white”, could be enough to destroy your credibility and so lead to refusal. An Immigration officer does not have a lot of data on which to decide so a mere hint you are being deceptive may be enough for rejection. Hence best to tell the truth. If that does not work at least you will have learned something for another occasion.

ii) Much of immigration control is to reduce the threat to the employment of nationals. Provided you have the means to sustain yourself, visiting is good (you put money into the economy) working not so much so (you take money out, even if adding value greater than that).

iii) VWP imposes much the same constraints as does a B-1/B-2 visa. Some business activities are allowed but full-time employment, even if short-term, is not.

iv) Gambling for a living is not an employment – you have no contract for or of services – and is not really depriving any national of employment.

v) 3 months in one year, or even 3 months every year does not seem like an occupation (assuming you are not an incredibly successful poker player). I’m guessing you have some other source of income during the other nine months of each year or just the rest of the time. Whatever that is would seem to be what most would consider your “occupation”.

I think best to answer along the lines “I’m a bit of a poker addict, I'm off to Vegas to lose my hard-earned cash, I think that counts as vacation”, perhaps with a wry grin. Seems truthful, not to jokey (Immigration has NO sense of humour) and might elicit some sympathy in your favour, should the person asking you ever themselves have played poker for money.

Also anyone who can afford to gamble for three months at a time creates an impression (with me) that they must be "of substance" and will not likely end up on support.