How many cupcakes can you decorate with three cups of frosting?? TIA

That depends on how much frosting you're gonna put on each one. 3 cups gives you 48 tablespoons, I'd estimate most decorations use 3-4 tablespoons, so I'd say maybe 12-16? Just an idea based on what I use. Hth.

Thanks

The last time I did a swirl I scraped it off and measured it. It was a little over 1 Tbsp for a 2D swirl, inside out rose type. The 2D looks like a 1M but (I think) takes less icing.