The behaviors your three year old are exhibiting could be a lack of attention. I know that I am not looking forward to this (I am expecting #2- due in the next 2 weeks). I have been told that if while you are breastfeeding the baby, you let your older child pick out a book, he sits next to you, and you read to him helps, or help him color while you nurse. Try getting a sling (one that will allow you to nurse with your hands free- (www.thingamasling.com is where I got mine, and it is wonderful- I can use it with my 2 year old too, or with my baby- it has many different positions you can use as the baby grows). Then you have both hands relatively free and can do other activities with your 3 yr old, you are still taking care of the baby, but you are also hands free for your three year old, and can give him a little more attention.
Have your husband take your son to the park, or do other things with him that are one-on-one, and then when you can, leave the baby home and you take your sone for a special one-on-one time as well. Also maybe talking to the friend's mom about when his friend's behavior started will give you an idea of where your son got the behavior from. Maybe spending less time with his friend will help his behavior, especially if you tell him that he can't go to his friend's house unless one behavior is stopped. Then when he stops the one behavior, then let him go back for a few days and do it again. Soon your son will realize that in order to go to his friend's house, he has to do XYZ, and has to act like a big boy. Ignoring the behaviors could help too- most times negative behaviors are just to get your attention, and if you don't give him the attention, then he will realize he has to do something else to get attention - use a rewards chart or something similar to reinforce good behaviors!
good luck and have fun! I can only imagine what lays ahead for me!