95 Geo Metro 2dr 3cyl. coolant always low

You most likely have a cracked head or bad head gasket.
The best way to determine is by looking all around the lower part of the head for any signs of water.
If there is no water leaking to the outside of the engine, The next step is to perform a compression test. The cylinders on a very good motor will be within about 7 pounds of compression or less ( if any cylinder is at 145LB, the highest or lowest would be around138-to-152)
The cylinders can be as far as 10% ( for a motor with 150, you could allow it to be as low as 135)
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What you are looking for is eather air coming out of the radiator, or, Two cylinders trading air, If you test the compression and you have two cylinders that are beside each other and they are lower than the other cylinder, Chances are they are trading air.