First of all, don't be so quick to call him lazy. Every kid has their "time" Here are some things to consider. He has a new baby sister! She is probably getting more attention than him.. including those diaper changes.. and if that gets him attention, it might be worth it to him to get it that way. My son was 3 when his first sister came along, and he did have a set back, but try to have some patience with him. See if one of you can take a week off from work so you are with him 24/7 to provide some security and consistency with the training (unless you can agree to a system.. you both are going to do it slightly different).
some tricks I learned with my son: peer pressure: ask some of his friends parents when they are together if your son can watch them go to the bathroom... at least for my son it did not help to see Daddy go... but once he saw his cousins or friends going, BINGO! For aim when he is standing up, give him a step stool so he can reach, the place one square of toilet paper on top of the water so it floats, ask him to make it sink, my son also liked to make the water turn green when I had a blue toilet cleaner in there. Some other incentives would be to keep a small jar of some small candies - like Skittles & M&Ms, give him 3 (because he is three years old, and ask him to count them out with you each time) for each time he goes in the potty. Another incentive is to have his very own calendar - or if your calendar is big enough, bet some small reward stickers like the teachers have (WalMart often sells pads of them and will have an entire sheet of small smiley face stickers about the size of the tip of your finger) each time he goes in the potty all day, put a sticker on that day, once he has a week of stickers, give him a reward, something you establish right now. For example, my 4 year old is wetting the bed again, if he wakes up dry, he gets the sticker, I have a picture of a toy he wants near the calendar, (less than $7) when he gets that week of stickers for staying dry at night, he gets the toy. Patience is huge.. if he sees you are angry or upset, that really won't help. Encourage him and give him hugs when he tries, you might feel silly jumping up and down or hollering "Yay!" and clapping your hands.. but your son will greatly appreciate your show of approval.. sometimes that is ALL it takes.. your positive attention, don't grumble too much over the mistakes, make a bigger deal out of his successes.
Hang in there.. if he is not finished by four.. that is okay... every kid is different.. I think my first son was completely potty trained by 4, so hang in there.
oh yeah.... pull ups.. they make them with designs that disappear if they wet them, and ones that will feel cool so they can recognize when they wet them. If you can find some in his size with a character or theme that he likes, that may also provide some motivation, my youngest son did not like the idea of peeing on spiderman or buzz and woody. So there are lots of options and "tools" out there to help you.
Good luck!