Make sure all the lines are hooked up correctly. It is going to take 3-8 hours to fill a 3 gallon RO storage tank. It depends on the capacity of the membrane (IE: 14, 20, 30, 50 or 75 gpd). It also depends on the TDS of the water. Water that has high TDS 500+ppm will take longer to produce product water than TDS of 100ppm. Check the pressure in the RO tank. When empty (In a 3 gallon tank) there should be 7psi pressure. A ten gallon tank takes 10psi. If it needs air you can use a bicycle pump to add appropriate air. Make sure the tank valve is turned off when adding air.
Do not use the RO faucet for several hours to allow tank to fill up. Also you should dump the first couple tanks of RO water to make sure any carbon fines are rinsed out.
RJ