pour coolant but it gets low too fast within 2- 3 weeks

You need to take it to a repair shop and ask them to perform a cooling system pressure test first. This way they will be able to pinpoint the location of the leak. Zee

If you have a coolant leak and don't see anything on the ground, then it sound like an internal leak. Coolant can leak into the engine and will turn the oil a milk color, or it is burning up in the engine from the head gasket. A test called a "block test" is performed to see if their is combustion gases in the cooling system. This test will confirm that you do or don't have a head gasket leak. We do that test here at Superior Automotive in Rancho Cordova, CA.