An almost 3 year old is going to push his sister sometimes, and is going to test you. That's what children that age DO. It's completely normal.
It is NOT normal for an almost-3yo to be able to sit still in time out. You are expecting way too much from him right now.
Instead of focusing on *punishing* him, I would instead reinforce what behavior is expected from him, and SHOW him. You are his teacher and you are there to help him learn correct behavior, you are not his enemy.
If he pushes his sister, you tell him, "No pushing. Pushing hurts sister. Be GENTLE with sister." and stroke his hand gently to show him what gentle touching feels like.
I don't know what other situations you are having trouble with, but testing limits is totally normal. That's how children learn about the world around them. Be gentle, yet firm.
I would really suggest you read "The Happiest Toddler on the Block" by Dr. harvey Karp. It is excellent at helping you communicate with your toddler in a way that he can understand, using very short, simple sentences.
Also, "Your Three-Year-Old, Friend or Enemy?" is a great book for understanding what is developmentally normal at this age. It sounds like your expectations are not in line for his age. Three year olds are pretty frustrating sometimes. That's just the way they are.
You definitely don't want to set up a situation where you are in a power struggle with him over everything. That doesn't help him OR you. You are both on the same team, right?
I would really focus on communication, and helping him learn *good* behavior, not just punishing him for what you think he is doing wrong.
You might also want to Google "positive discipline" for other methods to help handle specific situations.
My older son just turned 4, and this last year with him has been challenging. I have a 1yo too.
Every time that I focused on punishment with my 4yo, his behavior got worse, and our relationship suffered. Instead, I tried to stay patient with him and guide him towards correct behavior. Some days I repeated over and over, "No hitting brother. GENTLE hands with brother." or "That is a loud voice. We use quiet voices inside. If you want to use a loud voice, please go OUTSIDE." Or I would give him a snack and say, "Thank you Mommy," to let him know the polite way to respond to me.
But hey, you know what? It worked. He is very well-behaved now, very loving, and we kept our close relationship. He knows that Mama is here to help teach him how to behave nicely, not to punish him. We are working together, we are not enemies.
L. Hamilton
http://www.theportablebaby.com