3 level tablespoons = Approx. 25 g or 1 oz. Therefore, there are about 12 level tablespoons in 100 grams of flour.
For most home cooks, an additional gram or two of an ingredient won't make much of a difference. Once you start working with larger quantities, though, precision counts: An improper measurement can throw off a ratio and ruin a recipe.
Proper ingredient measurements are especially significant in baking. If you're working with larger amounts, especially in baking, seek out a precise conversion tool on the internet, or use a search engine to convert between metric and imperial.
Check out other useful cooking conversions at Allrecipes.co.uk.