A trio of online advertising vets are heading up a new, mobile advertising firm, Gradient X, which said this morning that it has raised $3.75M in a seed financing round. GradientX, led by Brian Baumgart and co-founded by Julie Mattern and Michael Lum, said the new funding round came from Rincon Venture Partners, GRP Partners, Crosscut Ventures, Walter Kortschak, Jim Willenborg, Clark Landry, Founder Collective, Double M Capital, Siemer Ventures and Baroda Ventures. Baumgart is the former Chief Strategy Officer at Adconion Direct, Mattern was co-founder and Chief Technologist at Rubicon Project, and Lum was Head of Engineering at OpenX. The firm said it will provide a demand side platform focused on advertising on mobile media.