You won't get your dream body overnight. Building muscle requires time and only patience will get you through the phases of demotivation and struggle. That being said, you can achieve A LOT in 3 months. Over the years I have seen guys putting on 5 lb, 10 or even 20 lb of lean muscle in 12 weeks. But even if you are not blessed with crazy genetics, like me, patience and discipline will get you a long way and make all the difference.
2. Compound Movements
Most beginners get demotivated because they hit the gym three times a week, improve their diet and still don't see any results. Almost always this happens because even though they train regularly, their bodies don't recieve the right stimulus to build more muscle. Only exercises that involve compound movements will make you bigger. The target several muscle groups at once and therefore improve your muscle interaction. If - for whatever reason - you can only do a handful of lifts, go with the essential compound exercises.
Muscles grow faster with an effective combination of training and nutrition. Look for protein rich foods, plenty of vegetables and unsaturated fats. Regular meals are also important, because in order to build muscle your body requires a constant supply of energy (=calories). Feeling hungry is an important sign that your body needs more food.
Your muscles not only need energy but a lot of water and nutrients, which must be supplied especially after your workout. Try to drink a gallon of water per day and cut alcohol and soda from your diet.
5. Rest & Regeneration
Science has shown that muscle growth can only occur when there is enough time for regeneration. You need at least 7 - 9 hours of sleep per night and should never train the same muscle within a timeframe of less than 72 hours.
6. Progressive Overload
In order to continuously build muscle you need to give your body an incentive to "change". This is done through an ever-continuing increase in training weights that your muscles have to adapt to. If you want to get bigger and stronger, you need to force your body to the increased stress (=weight) you are putting it through.