I am also targeting the Grand Slam(on fly). I do have quite a bit of information that I've been collecting over the past year of research on these three species, and specifically at this fishery. I've had several locations in mind prior to last summer, but Belize became my top choice for its great diversity in species alone.
Below is a link to one of many good articles that I've found over the past year that will help reinforce the notion that Belize, and more specifically Ambergris Caye, is one of only few places on earth that host "resident" Tarpon (year-round fish). Although the cows (#130+) you may be referring to are probably at their peak in late summer and early fall, there should be no shortage of mid-sized Tarpon all year long be on the flats, in the mangroves, or backwater. The key thing is finding the sweet spot for all three species if you are indeed focusing on all three.
Remember, the best source of information is your guide, regardless of where you intend on fishing. Ask plenty of questions, and then ask some more. http://www.elpescador.com/article1.html