One cubic metre per hour is 16.6667 litres per minute. Both units are metric, and the untidy factor comes from combining a thousand-fold volume step with a sixty-fold time step — 1,000 divided by 60.
About the units
Cubic metres per hour is the standard flow unit for ventilation, water supply and industrial process design. Litres per minute is used for smaller flows: taps, showers, small pumps and laboratory equipment. They describe the same quantity at different scales, and which one a specification uses is a reliable clue about the size of the system. A domestic tap runs at around 0.5 m³/h, an air handling unit for an office at several thousand.
The exact factor
The derivation is 1,000 litres per cubic metre divided by 60 minutes per hour, giving 16.6667. So 3 m³/h is 50 l/min, and 100 l/min is 6 m³/h. The relationship worth having alongside is to litres per second: 1 m³/h is 0.2778 l/s, and 1 l/s is 3.6 m³/h. Those three units cover almost all metric flow specification between them, and moving among them means keeping track of both the thousand and the sixty.
Where you meet this conversion
Building services, where the same system is specified in different units by different trades. A heat pump's water flow is given in m³/h by the manufacturer and set in l/min on the commissioning sheet. Water meters read in cubic metres while taps are rated in litres per minute. Pump duty points, heat exchanger specifications and filter flow ratings all cross the boundary. Industrial process design uses m³/h almost exclusively, so laboratory-scale results in l/min have to be scaled and converted together.