Help my 3 year old acting out

Idk if there is middle child syndrome but she's been acting so darn rotten it's crazy how impossible she is being I mean I scold her discipline her give her a little spanking don't seem to fade her I didn't have trouble with her sister when I was pregnant with her and she was same age when I was pregnant with second baby I need some suggestions or just be reassured that this to shall pass lol and to know if anyone else is in the same boat lol