This paper investigates the problem of triangular formation maneuver control for mobile 3-agent systems with bearing measurements. Different from controlling rigid formations' maneuvering by introducing a pair of mismatches per distance constraint, we introduce a pair of designed- mismatches per angle constraint which leads to the desired triangular formation shape. Considering that for the control of triangular formations with angle constraints, each agent aims at maintaining its own interior angle, to realize the formation maneuver control we design the mismatches into each agent's own desired interior angle. Two types of designed-mismatch are investigated: time-varying case and constant case. For the time-varying case, under the assumpti...