3 year old aggressive and defiant?

I have 3 kids, 3yo boy, 2yo girl and a 7 month old boy. My oldest, 3yo boy, has recently become very aggressive and defiant with everything. Anytime he gets even the slightest bit angry he holds his fists out and tenses his whole body up and shakes and grits his teeth. Just randomly throughout the day, he will just stiffen up and mumble words through gritted teeth and anger but nothing prompted it. When we get on to him we are immediate with discipline and he still is defiant and just ignores the command/request and does whatever it is anyways. He has always been so well behaved and willing to comply with our requests or not doing something he’s not supposed to. We have started him with one day a week mommy and me pre-k, he is about to start gymnastics one day a week, my husband and I have taken him on “date days” where it’s solely spent around him and his wants considering he is so young and has two even younger siblings. We make it a point to answer his questions first since he is fully speaking and the 2yo is just now starting to make out requests and things. We typically spend most of our days outside but with it being so cold the past month that has been almost nonexistent, so maybe he’s just not being able to fully expend his pent up engery… otherwise I’m at a loss. I’m a SAHM and learning to control my own emotions from a “hide your emotions, yell, spank, don’t cry” household in childhood. I don’t want to spank and I absolutely hate yelling so much but I’m at a loss because nothing else is working. Any advice, solidarity, and experience would make me feel better! Thanks if you’ve made it this far!