Basically, they were getting merked by the union, like the union has high ground and lee's army kept trying to go up the hill so the third day he tried to overwhelm the union by doing what is now called Pickett's charge where they charged through the union's heavy artillery but they still lost cause like half of the confederate army there died. This strategy was much more slipshod and improvised compared to his usually meticulous and advanced plans but it wasn't totally unpredictable because lee likes playing offense in war.