The first trimester is the most critical period in a kitten's life and an important time for their development and growth. At this stage, we need to give our kittens special attention and care to ensure that they grow up healthy and happy. Below, I will introduce you to some things that kittens should do in the first three months of development.

To begin with, we need to provide our kittens with enough love and companionship. Kittens are very dependent on humans in the first three months, and we should give them enough attention and care. We can spend more time with our kittens, interact and play with them, and let them feel loved and cared for by us. In addition, we can also groom the kittens to keep them neat and beautiful.

Furthermore, we need to offer the tiny cats proper and suitable food. Little cats need enough nourishment during their first three period to support their growth and development. We can choose cat food specially designed for young cats, which is rich in protein, vitamins and minerals. In the first trimester, we can give them wet or dry food and adjust the diet according to the amount of food and preferences of the kitten. In addition, we should also make sure that kittens have enough water to keep their water balance. 

Moreover, we need to take the cats to the vet regularly for health checks. A cat’s immune system is not fully developed in the first three months and is vulnerable to virus. We should follow the vet's advice and give cats the indispensable vaccinations to prevent common cat illnesses. Additionally, the vet can also help us check the physical condition of the cats to ensure that they are healthy and disease-free.

Overall, the first trimester of a young cat is an essential time for their growth. We need to provide them with a warm, safe and comfortable environment, offer them proper diet and take them to the vet for regular health checks.