Probably all three together would be best because there is different strains in each item. Kefir contains some strains of probiotic that are not in the pills.
If you ate just greek yogurt, it won’t come close to getting you where you want to be. But if you ate just kefir, I think it’d be quite beneficial.
If you get a human derived probiotic such as HMF neuro or megaflora, this won’t be found in the yogurt/kefir…one major plus I guess!
Also, once you get a kefir starting kit, you just add sugar and more milk, so it can be really cheap to use it.