Would it be good practice to include a personal statement letter along with you CV and cover letter ?
No, it's wasted effort and potentially detrimental.
What would a hiring manager think about it?
Any of the following:
- you aren't aware of hiring conventions
- you are any combination of: old1, antiquated or out of touch
- you can't follow instructions: most applications will tell you what they want: a cover letter and a resume, nothing else
Note that with all this, I don't even care what your "personal statement letter" is. If I'm hiring I want to see your resume and usually (though not always) I want to read a cover letter. I don't want a personal branding website, a portfolio that I didn't ask for, references that I didn't request, a gift, or anything else.
You say you want to "highlight your motivations in relation with your cursus, and the way you see professional life". I don't care. If I'm looking for a Java developer I want to see your resume to figure out what experience you have with it (at past jobs or in college). I want to know what interests you about the role and why you think you'd be a good fit, which I can read in your cover letter. The fact that you're applying to a full-time programming job in Java tells me that you want to continue your career in that role.
Send me your resume and a cover letter. I don't need anything else from you.
Will including this mean that I won't even look at your application? No. But this is a red flag for your application because it's something that excellent candidates simply don't do. You're associating yourself with a group of people who don't understand hiring conventions and that's a group that only rarely produces good employees. Graduates are somewhat of an exception to this as many don't know better, but even in that group following industry conventions will automatically put you in a separate category from the people who mail in a physical resume or call before being called.
1 - I'll clarify this since it's a controversial topic: it's not being old that's a problem when you're job searching, it's conforming to old stereotypes that you want to avoid. I don't have a problem hiring someone well into his fifties, I have a problem hiring someone who's too set in his ways to consider that he's going against conventions. Now I'm going to wonder if this person will struggle with new techniques or tools as well.