Welcome to the forum!
I went from bigger bikes to the R3, as have quite a few other riders/forum members, and do not regret it one bit. Every bike has its place, there is no end all be all bike, but for right now, until I can get really good with a smaller bike ( I measure this by maybe going from group C on the track, to group B), I think I will stay with the smaller lighweight class.
If you plan on only commuting and never going out to the canyons, or better yet, the race track, then yeah I see someone getting bored or tired of the R3 quickly because to them, fun is going fast in a straight line. For me though, and I think I speak for the majority of the riders, the real fun is in the twisties or on the race track. I am not saying the bigger bikes are not fun there, but the smaller bikes really are loads of fun in canyons or on the track, without having to deal with things that bigger bikes bring, such as higher cost to operate, own, and insure, slower learning (smaller bikes typically teach you proper riding technique at a faster rate, since they are more forgiving in some places, and force you to be perfect in other places, like carrying speed through a corner )
Ask your friends who have told you that you will get bored on your little 300 if they are bored yet doing this, or maybe they are bored because they cannot get to that level yet?
https://youtu.be/GKDN3okg4ko
FWIW I went from a 650 to a 600 SS to the R3. The 600 supersport was more fun than the 650, but the R3 truly is a fun bike to ride in the canyons and race track. I learned more on the R3 in a shorter amount of time than the other bikes. I was literally dragging knee within a week of owning the bike, something I have been striving to do with my previous bikes but could not do. Some people will say dragging knee isnt recommended on the street, it isnt a measure of skill, but to me it was an accomplishment I have been striving for.
For me, there are people at race tracks that do faster lap times on their R3/N300/RC390s than people on 600s and even liter bikes. Why is that? Because those are probably the guys that love riding so much, they didnt care what their friends thought of the little bikes and strove to ride better and gain more and more skill. All that skill can carry over when you think you truly are ready for the 600/650/liter bike/whatever.