Muay thai works great for losing weight and building muscle because it's an all-in-one deal: you're throwing punches for an hour straight, your heart's pounding at a thousand beats per minute, and at the same time you're working every muscle group - arms, core, legs, back. By 3 months you should already be noticing real differences in how you look and how you feel. The calorie burn is serious, way more than a lot of things people do at the gym.
Now, here's the thing nobody tells you: if you want to maximize muscle gains, you probably need to add some weight training too. It doesn't have to be much or replace muay thai - maybe twice a week of complementary strength work will give you better results than just technique and cardio alone. Muay thai is excellent for definition, burning fat, and building muscular endurance, but if your goal is real muscle growth, compound exercises with weights are still irreplaceable.
The other thing is nutrition. People train like crazy sometimes but eat whatever and then get frustrated when they don't see changes. If you eat well and stay consistent with muay thai - which you've already been doing for 3 months, that's solid - you're going to see results. But the combo of training + decent eating is what really closes the loop.