Is 300 points good improvement over three months?

I don't know what is points

300 points is really good if you didnt start at 100 and got to 400 if you did it the dishonest way by pretending to be so low when you started to get a rating boost and easy wins, but for most 300 is very good.

Yes, it's good, it shows you are learning from your mistakes in games and fixing them. A LOT of people keep making the same mistakes again and again without understanding what they're doing wrong, so they never improve much at all. Even though you are at a low rating, that improvement suggests that you have some talent for pattern recognition and are beginning to understand that the other player wants things too.

In my experience, you can continue exactly what you're doing learning from your games up to 800, then it's a good idea to learn a basic opening or at least opening principles, and some basic tactics (YouTube is very helpful). Work hard at not leaving your pieces available to be captured for free, and spotting when you can capture an enemy piece for free.

After 1,000 start asking yourself what the opponent wants after every time they make a move, and don't let them get what they want. After 1,200, learn about things like taking open files, outposts, blockading an enemy attack, and how to pawn storm.

As Calbitt said, you may not continue to see +100 per month progress, but then again you might! If you are young or have an analytical mind, and you stay motivated and play often, you can likely continue that rate of progress for quite a bit longer- I did