REALLY try to view the situation from your older child's view. When you are with your older child and your youngest cries, do you stop what you are doing and immediately run to the baby? If the answer is yes, even once, don't you think that your older child will have resentment issues. People talk all the time about how husbands will become jealous/resentful of their child because we moms tend to put everything aside for our kids, including our hubby.
You oldest was treated to all your attention, and suddenly it has been split, and more than likely it is unevenly. I am not poking at you, babies take more attention when they are awake. When your youngest is napping, do NOT take advantage to clean up, do the laundry, or anything housework unless you get him involved in a FUN way (yes, you can get yourself a little helper, if you create fun while doing it).
Really use your younger child's nap to give your older the time he wants and needs. Play a game with him (Go Fish, Memory, Old Maid, Candyland, etc) or fold your laundry while you watch a TV program and talk about it. Give him that special 1 on 1 time. I believe he desperately wants attention. When your younger one wakes up, do not stop what you are doing, but allow your younger child to entertain himself for a few moments while you finish up the game. Then announce that it is time to go get brother.
Do NOT tolerate any longer any acts of deliberate meanness. He must be told that this will not, nor does not happen any longer. You will have to punish him in a time out. Do not overfuss the baby if your older son just jumped over or threw something that did not hit, deal with your older one. Don't over dramatize what could have happened, your 3 year old does not understand that he could fall and land on his brother, rather give him a pillow that he can safely jump over (if jumping is allowed in the house).
When does he throw things? Is it when you are paying attention to your younger child? Is this his way of getting your attention focused back onto him? Remember, children that don't receive positive attention will seek attention through negative.
His starting pre-school needs to be given to him as a reward for his growing up, not viewed as a break for you. Assure him you will miss him, and talk about how big he has become. Do not think that they will deal with his being mean for you, because they do not have TIME, they will simply send him home and be viewed as a disruption to the other kids and their learning.
Odd as it may sound, really start to watch what he eats, and cut out the majority of his sweets. It is better for his health and then you can use them for his rewards. Smarties are a great treat, one roll will last more than 1 day, and they usually are a treat most kids like.
Also try to set aside special toys that are just for him, that the baby cannot touch or play with. Just a few, not a lot, but something that he can treasure.
You might want to consider using use one of those circle gated areas to confine your younger child. Explain to your older child that your younger child has space, as does he, and neither should interfere with the other.
Your older son might want to begin teaching your younger son words (something that might help him as well, since he has the pulling issue). Stress to him as the older brother, he is helping to teach his younger child. Maybe he would like to say words, and get that proud feeling we all get when the baby gets it. Maybe guide him with a word and tell him to see if the baby will say it. Don't make it a chore, just a suggestion that he has the right to refuse to do. If he wants to not play with his brother now, there is a huge difference in age; however, once you baby is 2 or 2 1/2, then your older one will find him much more fun.
Best of luck to you.