I want to pump gastric juice (can bethink as water) in flow rate of 3ml/min from reservoir
I've bought this pump
Dc6-12v r385 aquarium fish tank round water air dc diaphragm pump Sale - Banggood.com sold out-arrival notice-arrival notice
The flow rate spec on that pump is 1.5-2L / Min(approximately)
You may be able to put the output of the pump through a needle valve to cut the flow, but basically that pump is not appropriate for for your use.
I think some more research on small pumps is required. As @dave-in-nj suggests a peristaltic dosing pump may be the way to go.