Partners Mark Blackman, Doug Geer and Jon Sellew don’t exactly play around at their Sharon, Mass., startup Levelhead. (The seesaw is just an attractive art element.) The three do, however, attempt to keep “calm, cool and creative” at all times, which explains the shop’s name. The principals bring a mix of agency and client-side experience to their new venture, which is already handling copy, design and interactive assignments for Fidelity Investments, Caterpillar’s Preferred Mutual Funds and Genzyme.