It often takes shrubs several years before they begin to grow and bloom well. This is because the plants need time to properly adjust to their new site. If this is just the third year since planting, I wouldn't worry about slow growth unless the plant is also showing other signs of distress such as browning or yellowing foliage. An adage that applies to perennial flowers and shrubs is: The first year it sleeps, the second year it creeps, and the third year it leaps. I suspect that your magnolia will take off and grow well this year. Make sure your plant has good growing conditions (full sun and well-drained soil) and a site that's not too exposed. If your plant gets too little sun, is in heavy clay soil, or was planted too deeply, it may take longer to become established.