How bad is making mistakes in the lecture slides? I am giving a lecture in IT (for the first time) to more than 100 students. I refused to use the slides of the previous lecturer and I wanted to make my owns. They take me a lot of effort and I believe that are better than most lecture slides on the same topic.

The lecture includes so many mathematical equations (almost every slide). Because I am making them from scratch, I made already 3 mistakes in the equation (e.g. multiplication instead of addition). I discovered two of them during the lecture I said that it is a mistake. I corrected the mistakes before I uploaded the slides. 

the third mistake I did not notice but when I gave an example I used the correct one. In the exercise, the students were claiming that the equation does not give the same result as expected in the exercise (which is graded). I decided to give all the students the full note for that exercise.

My question is how bad to have these mistakes? Should I use the slides of the previous lecturer?
Everyone makes mistakes. Teachers, students, everyone. It isn't terrible that you make them, but it is good, and can be extremely instructive when you correct them. 

Your description suggests that you do the right thing here. Proof your slides and make corrections. But also, when mistakes are pointed out in class, not only admit them readily, but work through the correction as needed. If it is just a typing error then it is easy, but logic errors might require more effort. 

However, seeing a professor work in real time to solve a problem, rather than just showing prepared stuff is extremely enlightening. Students can get the idea when presentations are perfect that this stuff is supposed to be easy. In fact, it may not be easy and so seeing a solution develop, even seeing false starts and corrections, will give them more confidence in their own work. Especially if they struggle with some of the ideas. 

Your students will be evaluating you overall, not just your slides. If you act in a human and humane manner, especially by responding to their needs, you should do fine. But occasionally a student will be unreasonable. Don't worry too much about this. 

But perfection in a lecture isn't necessarily the most effect way to teach. That doesn't excuse sloppiness, of course, but consider the overall picture, not just the minute details. 

I've had students complement me more often on "lectures" that forced me to work "live" than on those that were very polished. In fact, I once tried to develop a "perfect" description of a hard problem and it left the student baffled. "Where did that come from?"

Be good. Very good. But recognize that you are human, which means being adaptable. 



As to using the previous slides, I have no opinion. They might be good, but they might be too perfect. They might also force you into stilted delivery, which you should avoid. 

But as a newcomer to this stuff, you might have conversations with colleagues about your teaching materials and get their feedback. People learn a lot by interacting with students, but it isn't obvious to a new teacher how that all fits together effectively. Some places have more senior instructors visit the classes of new faculty and then give feedback, either formally or informally. There is no real reason why you can't ask a trusted colleague to do that, even if not required. While it might feel risky, it also shows you are trying to improve your craft.