Not terrorists.
Why?
Point 1. The bombs were small (as seen on the pictures) and it seems very ineffective - Sure, three death and twelve badly wounded is terrible, and I feel sorry for them and their families, but driving a car into the crowd present there would have been more effective. Terrorist bombs would have been A. more effective and B. more deadly.
Point 2 - Weird places to put bombs. A Charity marathon? A library? A hotel? The first ones are 'public' places, and thus sounds terrorism more likely there - But that's kinda ruled out by point one. It seems the bomb placer just wanted to get as many blasts and attention as possible. There would be a lot of news reporters at the marathon, of course.
My guess some crazy psycho who teached himself to make bombs wanted attention. Either that, or this is all part of a plan to let people think it's just one crazy dude, and there's a big blast coming up. Conspiracy theories, go!