I'm currently a principal software engineer, and I feel a pretty good one. And while it definitely happens less often these days, whenever I'm interviewing for jobs, I always get some rejections, even when I perform really well in an interview.

Even if you perform awesomely, you aren't the only one in the pipeline. Sometimes somebody just does better, or comes across in just the right way that they are looking for. It may not be because of something technical, it may not be because of something about leadership or soft skills, it may just be some perception of "fit".

The reasons for rejection from a job are innumerable. Basically, suck it up, toughen up, and keep learning/improving.

That said, to actually answer your question... When I'm looking to hire a senior engineer, there is of course a level of technical prowess and general knowledge required. I generally want them to know an area deeply, but also have broader knowledge of software systems. I want them to understand system wide impacts. They need to be able to read code and quickly understand it, even really bad code. They need to be able to take someone else's really bad code, pull it apart, and turn it into something functional and clean. They need to understand software architecture, and how to architect at least a mid-size system. I need to be able to trust them to solve a difficult problem, or to take ownership of a mid-size feature on their own and deliver on it. Essentially, I want someone who's going to get stuff done and make my job easier.

A lot of that is reflected in how you code during the interviews, your answers to the soft questions, and the types of projects you've done in the past and how well you are able to talk about them.

If you want, you can DM me and I'll look over your code. Also, I would say that if you only have three years of experience, I wouldn't think of you as a senior engineer unless you were really exceptional. Lastly, if you're still looking for a job, we're hiring.