Mechanical controls are mostly two-variable system (input and output) with a clutch in between. The author has invented a new three variable mechanical rotary motion control system. The core building block of the system is a “transgear”. A transgear has three variables (input, control, and output) same as a transistor. A transgear is a mechanical three-variable gear assembly that just rely on the gear meshes and does not rely on cams or friction. Since transgears have three variables, they can control torque (τ) and angular velocity (ꙍ) separately and independently. The first product application of the core element was to a speed converter and the author named it “Hummingbird”. Speed converter Hummingbird is designed with two transgears and has three variables. Two transgear-controlled Hummingbird is a mechanical threevariable rotary motion control. Operationally, when the three variables are assigned with three functions, i.e., a variable input (first independent variable), a control input (second independent variable), and an output (dependent variable), the first transgear identifies the speed differences (delta (Δ) speed) between the variable input and the control input, and the second transgear eliminates the identified speed differences (Δ speed): therefore, the output of the speed converter Hummingbird becomes constant when the control input is constant. A properly designed Hummingbird is balanced with torque and speed, and the generated electrical power is greater than the control power used (electrical advantage). Hummingbird-controlled river turbine controls are scalable and can generate constant frequency baseload electricity with grid (grid-tied) or without grid (distributed generation). Speed converter Hummingbird can be used to harness Marine Hydrokinetic energy of rivers, tidal, wave, ocean current, and also wind energy.