Sedentary cats need about 50 kcal of metabolisable energy per kg of body weight. So an old 3kg cat would want about 150 kcal per day.
A 5ml scoop of Vetark Critical Care Formula supplies 18 kcal, so an old cat such as this would want 8 scoops a day to fill all his energy needs, but hopefully he would be taking food as well and wouldn't need that much CCF. Please discuss this with your vet, CCF isn't designed as a long term food, its a short term nursing food but it can also be used to enrich another diet.
CCF can be given by fine tube such as an oesophagostomy tube, many will take it directly, or the powder can be mixed into/onto other foods.