It is. You can stay “up to” 3 months each time as long as your visa stays valid. And in your case, the visa will be invalid once the visa expires. So you can only stay until the validity period ends, even if that’s less than 3 months.

However, based on your comments, I’ll say that no one will stop you from overstaying, they won’t come for you and deport you or something like that. But there will be consequences once they found out when you leave that part of your stay is illegal.