It depends on how potent the kratom is, and also how thick the grind is. Or is it crushed leaf? It might be worthwhile to get a scale. If it is crushed leaf I would say start with half of that, see how you feel after 45-60mins, and if it's weak stuff finish it off. If it's powder, start with half a tablespoon, wait an hour, if it's weak take a little more (certainly not 3 Tbs.)
If you like it, please never buy from a headshop and get it online. If you buy from headshops, you are contributing to the possibility of this wonderful plant being banned (It is often swept into the sae category as headshop RC's because they are sold side by side)